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LEPIDODENDRON    ACUMINATUM    (LESQUEREUX).     ONE-THIRD      NATURAL 
SIZE.    FOUND  AT  WORCESTER  "COAL"  MINE,  1883.    PHOTO- 
GRAPH BY  JOHN  M.  BLAKE. 


\v 

THE 


GEOLOGY  OF  WORCESTER, 

MASSACHUSETTS. 


JOSEPH  H.  PERRY, 
ill 


or  WORCESTER, 


BENJAMIN  K.  EMERSON, 

PROFESSOR  OF  GEOLOGY  IN  AMHERST  COLLEGE. 


PUBLISHED  BY  THE  WORCESTER  NATURAL  HISTORY  SOCIETY, 
WORCESTER,  MASS. 


Copyright,  1903, 
Jy  the  Worcester  Natural  History  Society. 


WORCESTER,  MASS. 
1-KK8S  OF  CHA8.  HAMILTO 


The  printing  of  this  volume  was  authorized  by  the 
Worcester  Natural  History  Society  at  a  regular  meeting 
held  April  30,  1902. 

FRANKLI.V  P.  RICE, 
HERBERT  D.  BRAMAN, 
HENRY  BILLINGS, 
JOSEPH  H.  PERRY, 

Committee  of  Publication. 


TABLE  OF  CONTENTS. 


PAGE. 

Prefatory  note xiii 

Preface  xv 

Introduction 1 

I. 

WORCESTER  PHYLLITE  AND  MICA  SCHIST. 

Rock  in  Court  House  hill 3 

Rock  in  Elm  and  Pleasant  streets,  in  Oread  hill  and  Woodland  street .  .  4 

Extent  of  t*he  phyllite 4 

Study  of  rock  at  the  deep  cut  of  the  B.  &  A.  R.  R 5 

Detailed  study  of  rock  at  the  deep  cut  of  the  B.  &  A.  R.  R 6 

Andalusite    phyllite 6 

Development  of  secondary  structure 7 

Micaceous  quartzite 8 

Dike  of  aplite 9 

Weathering  of  the  rock  of  the  cut 10 

Hornblende  rock,  a  basic  eruptive,  in  the  cut 11 

Glacial  striae  on  surface  of  the  ledge 12 

Observations  in  Plantation  street 12 

Granite  porphyry  in  bowlders 13 

Approaching  the  " Coal  Mine" 14 

"  Coal  Mine"  rock 14 

Iron  pyrites  in 15 

Graphite  at  " Coal  Mine"    15 

Graphitic  acid  made  from  graphite  of  " Coal  Mine"    16 

Graphite  breccia 16 

Graphite  phyllite  beneath  graphite  breccia 17 

Fibrous  prochlorite  at  " Coal  Mine" 17 

Fossils  at  " Coal  Mine" 18 

Former  state  of  phyllite    19 

Metamorphism  in  formation  of  phyllite    19 

Agents  of  metamorphism, 

Pressure,  Heat,  Moisture  20 

Uniformity  in  position  of  mica  scales  and  reason  therefor    21 

Original  layers  of  the  phyllite 22 

Phyllite  at  the  Summit   23 

Folding  of  quartz  veins 23 

Double  carbonate  of  iron  and  calcium  .  .  .24 


VI  TABLE   OF  CONTENTS. 

PAGE. 

Micaceous  quartzite  or  quartzose  mica  schist  at  Summit 24 

Crumpling  in  micaceous  quartzite 24 

Difficulty  in  estimating  thickness  of   25 

Phases  of  phyllite, 

Finely  ribbed  phyllite 25 

New  structure  in ^ 26 

Mica    schist,  garnetiferous .' 26 

Andalusite  phyllite 27 

Chiastolite  schist 28 

Age  of  Worcester  phyllite 28 


II. 

WORCESTER  QUARTZITE. 

Description  of •. 30 

Micaceous  quartzite  beneath  Newton  hill 31 

"                 "        in  Grove  street 31 

Origin  of  micaceous  quartzite    32 

Fault  in                               "          in  Grove  street   33 

Quartzite  in  Dodge  park 34 

Extension  of  quartzite 34 

Phyllite  between  Dodge  and  North  parks 35 

Crushed  granite  in  North  park 35 

Micaceous  quartzite  in  North  park 36 

"       in    Lincoln    street 37 

Calcite  in  quartzite  in  Lincoln  street 37 

Fine  quartz  veins  in  quartzite  in  Lincoln  street 37 

Variation  in  strike  of  the  quartzite  in  Lincoln  street 37 

Quartzite  on  Green  farm 38 

Quartzite  in  East  Kendall  street   39 

Relation  of  granite  to  quartzite  in  East  Kendall  street 39 

Quartzite  at  corner  of  Hunt  and  Shrewsbury  streets 40 

Breccia  near  Shrewsbury  street  40 

Relation  of  breccia  to  quartzite 41 

Quartzite  in  "  Coal  Mine "  area 41 

Extension  of  Millstone  hill  area  to  Oakdale    42 

Oakdale  anticline 43 

Millstone  hill— Oakdale  anticline    44 

Folding  in  micaceous  quartzite  at  Wigwam  hill 44 

Fault  or  warping  in  structure  of  quartzite  in  "Coal  Mine"  area 47 

Relation  of  quartzite  and  phyllite  bands 48 

Thickness  of  the  phyllite 49 

Age  of  micaceous  quart  zite 50 


TABLE    OF   CONTENTS.  Vll 

HI. 

MILLSTONE  HILL. 

PAOE. 

Hill  of  granite 52 

Description  of  granite 52 

Feldspar,  monorlinic  and  triclinic 53 

Quartz,  smoky,  blue,  amethystine 53 

Quartz  crystals  in 54 

Biotite  in 54 

Purple  fluor  spar  in    55 

Proofs  that  it  is  granite 55 

Disturbance  of   neighboring  rocks 56 

Inclusions  of  phyllite  in    58 

"          "    micaceous  quartzite 59 

Solidified  beneath  the  surface 00 

Possible  method  of  measuring  the  depth  at  which  the  granite  soli- 
dified    00 

Aplite  dike  in  granite  of  Millstone  hill 02 

Formation  of  aplite  dikes 64 

Inclusion  of  phyllite  in  aplite   05 

Dark  lines  in  aplite    .65 

Oxidized  porous  border  of  aplite    00 

Quartzite  in  aplite 66 

Aplite  breccia   66 

"        having  an  odor 67 

Minerals  found  in  the  granite, 

Beryl 67-68 

Garnet ; 68 

Sphalerite 68 

Molybdenite 68 

Purple  fluor  spar 69 

Segregated  mass  of  minerals 69 

Magma  a  complex  chemical  solution 69 

Effect  of  slow  cooling  on  magma 70 

"  pressure           "       "       71 

Crystallization  of  magma 71 

Ankerite  in  granite , 72 

Vein  minerals  in  granite    73 

Quartz  crystals 73 

Green,  purple,  white  fluor  spar 73 

Iron  pyrites 73 

Dark  grey  variety  of  granite 73 

Phyllite  highly  impregnated  with  solutions  from  granite   74 

Jointing  of  granite 75 

Crushing  and  production  of  foliated  structure  along  joint  planes    76 

Weathering  of  granite • 76 

Disintegration  due  to  fluor  spar  and  iron  pyrites 77 

Summary  of  facts  in  regard  to  Millstone  hill 78 


Vlll  TABLE  OF    CONTENTS. 

IV. 

BOLTON  GNEISS. 

PAOE. 

Southeast  slope  of  Wigwam  hill 79 

Ballard's  quarry  near  Quinsigamond    80 

Study  of  rocks  of,  band  by  band    80 

Gneissoid  granite 80 

Hornblendic  mica  schist  or  gneiss    81 

Gneissoid  granite 82 

Biotite  schist  resembling  conglomerate    82 

Hornblendic  biotite  gneiss  or  schist 82 

Light  grey  granite 83 

Gneissoid  granite 84 

Schist  containing  pebble-like  inclusions 84 

Gneissoid  granite 84 

Fine  grained,  brownish  grey  granite 85 

Gneissoid  granite 8(1 

Biotite  schist  and  gneissoid  granite 80 

Gneissoid  granite 86 

Grouping  of  the  bands v 86 

Time  relation  of  the  rocks 86 

Origin  of  mica  hornblende  schist 87 

Mica  schist  a  pseudo-metamorphic  conglomerate    88 

Chloritic  biotite  schist  also  a  pseudo-metamorphic  conglomerate   89 

Position  of  the  laminae  and  bands  an  evidence  of  folding 89 

Small  folds  in  a  plane  at  right  angles  to  that  of  the  large  folds 90 

Condition  of  the  rock  at  time  of  folding    91 

Difficulty  in  estimating  thickness  of  the  schists .' 91 

Possible  cause  of  folding  of  schists 92 

Connection  between  sheets  of  granite ' 92 

Brief  review  of  history  of  rocks  of  quarry 93 

Quarry  a  mineral  locality 93 

Zircon 93 

Allanite 94 

Hornblende 94 

Amber  colored  garnets 94 

Garnetiferous  mica  schist    9,5 

Graphite  in         "           "        95 

Black  hornblende  in  mica  schist    95 

Garnetiferous  gneiss 96 

Calcite 96 

Quartz  crystals 96 

Chabazite 97 

Stilbite    97 

Prehnite    97 

Pyrrhotite 98 

Kpidote 99 

Vermiculite  .  .  .99 


TABLE   OF    CONTENTS.  IX 

PAGE. 

Prochlorite 99 

Iron  pyrites 100 

Copper  pyrites 100 

Malachite 100 

Azurite    100 

Actinolite 100 

Quarry  a  hand  specimen  of  a  large  area 100 

Fibrolite  mica  schist 100 

Millbury  limestone   101 

Minerals  in  limestone 102 

Graphite 102 

Actinolite 103 

Other  minerals 103 

Extent  of  limestone 103 

Other  limestone  areas  in  Bolton  gneiss 104 

Origin  of  limestone 104 

V. 

SHREWSBURY  DIKE. 

Rusty  mica  schist  with 105 

Hornblende  schist    105 

Second  outcrop 1 06 

Third  outcrop  107 

Description  of  this  phase  of  rock  of 108 

Name  of  rock    108 

Pyrrhotite  in  diorite 108 

Chalcopyrite 108 

Scapolite 109 

Magnetite   109 

Fourth  outcrop 109 

Description  of  rock  of   109 

Olivine  in 110 

Dolomite  in 110 

Tremolite  in    110 

Fibrous  talc Ill 

Rock  a  massive  talc Ill 

Fifth  outcrop Ill 

Sixth         "       112 

Seventh    "       112 

Scapolite 112 

Rusty  mica  schist  with 112 

Eighth  outcrop 113 

Ninth        "       113 

Tenth        "       ..." 114 

Relation  of  the •<(>  ten  outcrops 114 


X  TABLE    OF    CONTENTS. 

VI. 

ROCKS  IN  THE  BALLARD  FIELD  AT  QUINSIGAMOND  AND  RELATION 

OF  BOLTON  GNEISS  TO  THE  CARBONIFEROUS  ROCKS. 

PAGE. 

Problem   116 

Location  of  field 1 16 

Granite  of      " 116 

Aplite  dikes  in  granite 117 

Glacial  marks  on .- 118 

Dike  of  hornblendic  rock  in  granite 118 

Outcrops  of  hornblendic  rock  in  neighboring  fields 118 

Outward  appearance  of  the  diorite    119 

Appearance  of  diorite  on  fresh  surface    119 

Garnet  in  diorite 120 

Diorite  on  Heywood  farm     '. 120 

Relation  of  diorite  outcrops  to  each  other    120 

Garnetiferous  hornblende  granite   121 

Metamorphic  rocks  of  Ballard's  field 122 

Mica  schist 122 

Micaceous  quartzite 122 

Mica  schist  between  Bolton  gneiss  and  micaceous  quartzite    123 

Relation  of  micaceous  quartzite  to  mica  schist    124 

Mica  schist  and  micaceous  quartzite,  Carboniferous 127 

Relation  of  Bolton  gneiss  to  Carboniferous  rocks 128 

Comparing  mica  schist  of  Bolton  gneiss  with  micaceous  quart/ite  129 


VII. 

PAXTON  AND  BRIMFIELD  SCHISTS. 

Location  of  Paxton  schist 130 

Tourmaline   granite 130 

Quartzose  mica  schist     130 

Tatnuck    hill    quarry 131 

Description  of  schist  of 131 

Variation  in  Paxton  schist   132 

Tourmaline  granite  in 132 

Relation  of  Paxton  schist  to  edge  of  plateau  of  Central  Massachusetts  134 

Glacial  marks  on  quarry  ledge 134 

Other  minerals  at  the  quarry 1 34 

Granite  area  south  of  Fowler  street 135 

Granite  widely  distributed  in  Paxton  schist 136 

Extension  of  Paxton  schist  to  south 136 

"  "         "  "         north    .  .136 


TABLE   OF  CONTENTS.  XI 

PAGE. 

Relation  of  Paxton  schist  to  Carboniferous  quart zite 137 

Ledge  in  Chandler  street     137 

Tourmaline    granite   in  137 

Apatite    in 138 

Ledge  in  Mill  street    138 

Paxton  schist  a  coarser  phase  of  Carboniferous  micaceous  quart /ite  139 

Paxton  schist  and  Bolton  gneiss  equivalents 139 

Pseudo-conglomerate  of  Paxton  schist 1-10 

Difference  between  Paxton  schist  and  Bolton  gneiss 140 


BRIMFIELD    SCHIST. 

Described 141 

Minerals  in 142 

Extension  of   142 

Relation  of  Brimfield  and  Paxton  schists  in  Tatnuck  hill 143 

Relation  of  Brimfield  and  Paxton  schists  at  the  Cascade    143 

Remnants  of  Brimfield  schist 144 

Remnant  of  Brimfield  schist  in  hillside  at  end  of  North  Bend  street.  .  .  145 

Position  of  the  schists  in  this  hillside 145 

Illustration  explaining  this .  146 

Relative  positions  of  these  two  schists    146 

Small  anticlines  in  this  hillside 147 

Tourmaline  granite  in  these  schists  147 

Faults  in  this  side  hill    147 

Age  of  Brimfield  schist    148 

Paxton  and  Brimfield  schists  the  rocks  of  the  Plateau  of  Central  Mass- 
achusetts    149 

Graphite  mine  at  Sturbridge    150 


VIII. 

GENERAL   GEOLOGY  OP  WORCESTER   AND  OF  THP    PLATEAU   OF 
CENTRAL  MASSACHUSETTS. 

Plateau  of  Central  Massachusetts  defined '.  .  1 .51 

Rivers  of  the  plateau 151 

The  plateau  a  peneplain    151 

Situation  of  Worcester  in  the  plateau 152 

Worcester  phyllite  as  related  to  the  plateau 152 

Carboniferous  quartzite  in  the  plateau 152 

Granite  in  the  plateau 153 

Bolton  gneiss  in  the  plateau   1 54 

Diorite  in  Bolton  gneiss  area    154 

Extent  of  the  Bolton  gneiss    155 

Best  direction  in  which  to  trace  the  rocks  from  Worcester 155 

Westboro  quartzite  in  the  plateau 155 


Xll  TABLE   OF    CONTENTS. 

PAGE. 

Anticline  in  southeastern  border  of  the  plateau 156 

Northbridge  gneiss  in  the  plateau    156 

Trace  the  rocks  westerly  from  Worcester 156 

Paxton  schist  in  the  plateau .  157 

Brimfield  schist  in  the  plateau  157 

Extension  of  the  Carboniferous  rocks 158 

Age  of  the  plateau 158 

Juratrias  land  surface  of  the  plateau  region 158 


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PREFATORY  NOTE. 


THE  work  of  choosing  the  material  to  be  presented  in  this  work 
and  of  writing  out  the  descriptions  and  preparing  the  illustra- 
tions, has  fallen  wholly  to  Mr.  Perry,  while  the  subscriber 
has  gone  over  the  work  carefully,  and  helped  in  suggestion  and 
revision. 

The  writer  has  been  engaged  for  several  years  in  the  mapping 
of  the  rocks  of  Worcester  County  for  the  U.  S.  Geological  Sur- 
vey, and  Mr.  Perry  has  assisted  him  in  this  work  from  its 
beginning.  He  has  come  thus  to  have  a  very  full  knowledge 
of  the  geology  of  the  county  and  especially  of  the  region  about 
Worcester.  The  work  thus  incorporates  much  work  that  has 
been  done  under  the  authority  of  the  U.  S.  Geological  Survey, 
and  is  published  in  this  form  by  permission  of  the  Director. 

B.  K.  EMERSON. 


PREFACE. 


THIS  book  has  not  been  written  for  geologists.  It  has  been 
written  for  the  people  of  Worcester;  for  those  who  have  no 
technical  knowledge  of  the  subject  of  geology,  and  in  such  a 
style,  we  hope,  that,  taken  in  the  hand,  it  will  serve  as  a  guide 
over  the  fields  and  through  the  streets  in  the  study  of  the  rocks 
of  Worcester,  and  of  their  relations  to  one  another.  But  while 
the  book  is  written  for  the  amateur  in  nature  study,  rather 
than  for  the  professional  geologist,  we  have  not  tried  to  slide 
over  or  omit  the  problems  here  presented,  but  to  solve  them  in 
untechnical  language. 

To  the  people  of  Worcester,  then,  we  dedicate  this  book, 
hoping  that  it  may  be  a  source  of  pleasure  and  profit  in  their 
walks  about  the  city. 


MAP  OF  DRUMLINS  OF  WORCESTER. 
MADE  FROM  TOPOGRAPHIC  MAP  OF  U.  S.  GEOLOG- 
ICAL SURVEY. 


INTRODUCTION. 


IN  the  "  Physical  Geography  of  Worcester"  it  has  been  pointed 
out  that  beneath  all  of  the  loose  rock  material  constituting  the 
larger  part  of  the  earth's  surface  about  us,  there  is  a  rock-floor ; 
that  this  loose  material  covering  this  rock-floor  is  of  varying 
thickness — frequently  not  more  than  twenty-five  feet  deep,  but 
in  the  drumlins  it  reaches  a  thickness  of  150  to  250  feet,  and 
in  the  sands  and  gravels,  partially  filling  the  old  brook  and  river 
valleys,  it  is  100  to  200  feet  in  depth;  that,  here  and  there, 
parts  of  this  rock-floor  appear  at  the  surface,  and  these  parts 
are  called  outcrops  or,  in  common  language,  ledges.  Whenever 
we  see  a  ledge,  we  are  to  think  of  it  as  a  visible  part  of  this 
rock-floor  extending  in  all  directions  around  beneath  this  thin 
coating  of  loose  material.  Whenever  the  workman,  in  digging 
the  sewer  trench,  reaches  ledge,  he  has  simply  excavated  down 
to  the  underlying  rock-floor.  The  elevations  and  depressions 
in  this  rock-floor,  within  Worcester,  agree  only  in  a  very  gen- 
eral way  with  the  elevations  and  depressions  of  the  visible 
surface  of  the  earth.  This  is  especially  due  to  the  abundance 
of  drumlins  in  the  western  part  of  Worcester ;  and,  as  these  are 
a  part  of  the  covering,  we  must  annihilate  these  in  thought 
while  we  think  of  the  rock-floor.  To  make  this  easy  the  accom- 
panying map  has  been  added  showing  just  which  of  our  hills 
are  drumlins. 

If  then  we  wish  to  study  this  rock-floor,  we  must  study  it  as 
it  is  revealed  in  its  outcrops  or  ledges,  both  at  the  surface  and 
in  excavations  made  by  workmen. 


GEOLOGY  OF  WORCESTER. 


CHAPTER  I. 
WORCESTER  PHYLLITE  AND  MICA  SCHIST. 

To  one  who  has  not  studied  the  ledges  in  any  systematic  way, 
but  has  only  casually  observed  them  here  and  there,  while  riding 
or  walking  about  the  city,  they  may  seem  to  be  distributed  in 
the  most  confused  manner.  While  there  is  quite  a  remarkable 
variety  for  so  small  an  area,  the  ledges  are  arranged  in  a  well  de- 
nned order  which  clearly  appears  when  we  study  them  systemati- 
cally, and  fix  their  positions  on  the  map.  It  makes  little  difference 
where  we  commence  in  our  study  of  the  rock-floor  as  it  appears 
in  the  outcropping  ledges,  for  everywhere  there  is  order.  Let  us, 
then,  begin  at  a  point  as  near  as  possible  to  the  Natural  History 
Society's  building.  When  the  workmen  excavated  the  cellar  of 
Rock  m  ^ne  new  courthouse  on  Court  Hill,  they  cut  into 
court  House  the  underlying  ledges,  and  revealed  the  rock-floor 

Hill 

for  our  study.  The  rock  is  of  a  slaty  drab  color; 
has  a  noticeably  smooth,  greasy  feel;  is  irregularly  laminated,  and 
hence,  because  of  the  cleavage  between  the  laminae,  tends  to  break 
in  flaky  fragments,  a  mass  of  these  looking  like  a  pile  of  slaty  chips. 
The  laminae  of  this  rock  are  very  thin — only  a  small  fraction  of 
an  inch  in  thickness — and  are  roughly  parallel,  and  irregularly 
curved;  hence  the  unevenness  of  the  surface  when  the  laminae  are 
separated  as  they  are  when  the  coarse  irregular  flakes  are  chipped 
off.  While  the  rock  breaks  quite  easily  between  the  laminae  be- 
cause of  its  cleavage,  at  right  angles  to  them  it  is  quite  hard  and 
resisting.  On  the  surface  of  the  laminae  there  is  a  faint  lustre 
or  glimmer,  except  where  weathering  has  taken  place,  producing 
a  coating  of  iron  rust.  Such  a  rock  as  this  is  now  called  phyllite. 
It  was  formerly  called  an  argillite,  but  this  name  implied  the  pres- 
ence of  clay  in  the  rock.  The  original  mud  rock  or  shale,  out  of 
which  it  was  formed,  has  been  recrystallized  into  a  mat  of  minute 
mica  scales  and  quartz  grains.  When  the  rock  becomes  so  coarsely 


4  GEOLOGY   OF   WORCESTER. 

crystalline  that  the  mica  scales  are  clearly  visible,  it  is  called  a  mica 
schist.  Such  is  the  rock-floor  here.  With  this  general  descrip- 
tion in  our  minds,  let  us  extend  our  observations.  Some  years 
ago,  while  workmen  were  digging  a  trench  through  Lincoln  street, 
all  the  way  up  the  hill  they  found  rock  of  this -same  description; 
at  Adams  square  there  are  numerous  outcrops  or  ledges  of  the 
same;  at  the  Summit  station  of  the  Boston  and  Maine  railroad 
is  a  deep  cut  through  the  same  kind  of  rock.  We  may  conclude, 
with  a  reasonable  degree  of  certainty,  that  the  underlying  rock- 
floor  between  these  points  is  made  up  of  like  rock. 

If,  on  the  other  hand,  we  extend  our  observations 
Rock  in  Elm       ^o  ^he  south,  we  find  that  the  same  rock  appeared 

and  Pleasant 

streets,  oread  as  workmen  dug  the  sewer  trench  in  Elm  street  near 
Fruit,  and  in  Pleasant  near  Merrick  and  Russell 
streets;  it  also  appears  in  large  outcrops  in  Oread  Hill, 
and  in  various  places  in  Woodland  street  to  Clark 
University.  On  passing  to  the  eastern  part  of  the  city,  and  starting, 
in  our  observations,  at  the  deep  cut  where  the  Boston  and  Albany 
railroad  passes  under  Plantation  street,  we  find  this  same  phyllite 
exposed  to  a  depth  of  twenty-five  feet  and  for  a  distance  of  a  third 
of  a  mile  or  so.  Thence  we  may  follow  this  rock,  in  numerous  out- 
crops, south  through  Plantation  street  over  Oak  Hill,  and  thence 
over  Providence-street  and  Vernon-street  hills,  across  the  Blackstone 
to  Pakachoag  Hill  and  thence  into  Auburn.  Reasoning  as  we  did  in 
the  case  of  Court  Hill,  Lincoln  street  and  Summit  areas,  we  conclude 
that,  under  the  covering  of  loose  material  consisting  of  sand,  gravel, 
clay  and  the  like,  this  phyllite  extends,  connecting  these  separated 
areas.  That  we  make  no  mistake  in  this  conclusion  is  shown  by 
numerous  borings  and  cuttings  which  have  revealed  the  surface  of 
the  rock-floor  between  some  of  these  localities.  By  just  such 
work  and  reasoning  has  the  accompanying  geological  map  been 
made  out,  showing  that  part  of  Worcester  under  which  this  phyllite 
extends,  constituting  the  rock-floor.  It  is  the  area  marked  blue. 

From  this  we  see  that  the  area,  within  which  this 
roc^c  *s  f°und,  extends  across  Worcester  in  a  north- 
south  direction,  varies  in  width,  and  is  quite  irregu- 
lar in  outline.  But  this  study  has  not  been  confined  to  Worcester. 
In  like  manner  this  rock  may  be  traced  far  to  the  south,  where 
it  becomes  a  mica  schist,  through  Auburn,  Oxford,  Dudley,  thence 
into  Connecticut  through  Woodstock  and  Pomfret;  and  there,  about 


RAILROAD  CUT  NEAR  BLOOMINGDALE. 


GEOLOGY    OF    WORCESTER.  O 

a  mile  north  of  Wolf  Den  Hill,  famous  because  of  Gen.  Israel 
Putnam's  undaunted  courage,  this  rock  conies  to  an  end.  Through 
this  extent  it  narrows,  until  it  is  not  more  than  200-300  feet  wide 
at  its  southern  end. 

In  like  manner  it  has  been  traced  to  the  north  where  it  broadens 
through  Boylston,  West  Boylston,  Sterling,  Lancaster  and  other 
towns  still  farther  to  the  north. 

Having  thus  seen,  in  a  general  way,  somewhat  of 
study  of  the  the  extent  of  this  rock  underlying  Worcester,  let  us 
iiee^utotthe  attempt  to  obtain  a  clearer  idea  of  it  from  a  more 
B.  &  A.  R.  R.  careful  study  at  some  of  its  typical  localities.  There 
is  no  locality  where  the  rock  may  be  studied  to 
better  advantage  than  at  the  deep  cut  through  which  the  Boston 
and  Albany  railroad  passes  under  Plantation  street.  Here,  for 
about  a  third  of  a  mile,  a  cutting  has  been  made  through  the  solid 
rock-floor  to  the  depth  of  twenty-five  feet  or  more. 

As  we  look  at  the  solid  rock  on  either  side,  we  notice  a  certain 
regularity  of  structure.  The  rock  material  is  arranged  in  thin 
sheets.  This  becomes  even  more  evident,  if  we  break  a  piece 
from  the  vertical  wall.  Under  the  blow  of  the  hammer,  where 
the  rock  is  somewhat  weathered,  it  may  break  into  a  mass  of 
thin  sheets  or  laminae;  or  if  we  look  at  a  surface  at  right  angles 
to  the  sheets,  we  may  see,  in  the  thickness  of  only  an  inch,  so 
many  laminae  that  we  can  with  difficulty  count  them.  More- 
over these  sheets  or  laminae  have  a  certain  parallelism  in  position, 
standing  up  at  a  high  angle  from  a  horizontal  position  and  pointing 
in  a  northerly  direction.  To  be  more  accurate  we  determine  this 
direction  in  several  places,  by  means  of  the  compass,  remembering 
that  the  magnetic  needle  here  points  about  twelve  degrees  west  of  the 
true  north,  and  find  the  direction  in  which  the  laminae  point, 
to  be  generally  between  twenty  and  thirty  degrees  east  of  north. 
This  is  called  the  strike  of  this  rock;  and  the  strike  of  rocks  will 
frequently  be  taken  in  our  study,  for  it  will  reveal  to  us  many 
facts  that  would  not  otherwise  be  observed.  We  also  notice  that 
the  laminae  slant  down  towards  the  west.  We  measure  the  angle 
which  they  make  with  a  horizontal  plane,  and  find  this  to  be, 
generally,  about  eighty  degrees.  This  is  called  the  dip  of  the  rock; 
and  because  the  slant  is  towards  the  west,  we  say  that  the  dip  is 
to  the  west.  This,  also,  we  must  carefully  note  in  our  observa- 
tions, if  we  would  comprehend  aright  the  facts  presented  to  us. 


6  GEOLOGY   OF  WORCESTER. 

Looking  at  the  surface  presented  when  the  rock  breaks  between 
the  laminae,  we  see  that  the  rock  is  of  a  light  slaty  color.  It  glistens 
brightly  in  the  sunlight.  This  glistening  is  due  to  an  infinite 
number  of  bright  points,  each  one  of  which,  under  the  magnifying 
glass,  is  seen  to  be  a  little  mica  scale.  The  surface  is  a  mat  or  felt 
of  these  scales,  each  parallel  in  position  with  its  neighbors.  It 
is  because  of  this  parallelism  that  the  rock  material  is  arranged  in 
laminae,  and  breaks  into  these  thin  sheets.  It  breaks  more  easily 
between  the  scales  than  it  does  across  them.  The  color  of  the 
ledge  varies  considerably.  Seen  from  the  car  window,  one  might 
think  the  whole  ledge  to  be  of  a  very  dark,  dull,  dirty  grey  color; 
but  in  reality  there  are  many  shades  of  red  and  brownish  yellow,  of 
dark  brown  and  grey,  all  more  or  less  modified  by  the  accumula- 
tion of  smoke  and  cinders  from  the  engines. 

But  if  we  give  only  this  casual  examination  to  the 
Detailed  study  rocks  here,  they  all  appear  much  the  same.  In  reality 
°the1cut>fromf  tnev  are  quite  variable.  Let  us  therefore  examine 
east  to  west,  the  ledge  carefully,  and  every  few  feet  break  off  frag- 
ments for  examination.  Commencing  at  the  east  t-nd 
of  the  cut,  we  break  a  fragment  from  the  ledge,  where  it  just  appears 
above  the  ground.  It  is  a  fissile,  crinkled  phyllite  of  a  dark 
slate-drab  color,  much  darker  than  is  the  average,  and  of  a  greasy 
feel.  Drawn  across  the  page,  it  leaves  a  mark  like  that  left  by 
the  lead  pencil.  We  immediately  know  that  this  contains  graphite, 
the  substance  contained  in  the  pencil.  To  this  graphite  is  due  in 
part,  at  least,  the  smooth  greasy  feel.  This  feel  is  in  part  also  due 
to  the  decomposed  mica  of  the  phyllite.  Such  hydrated  mica  is 
often  called  sericite,  and  the  rock  a  sericite  schist.  Here  may  also 
be  noticed  masses  of  white,  granular,  glassy  quartz  which  are  parts 
of  quartz  veins.  In  this  rock,  at  some  time,  fissures  or  cracks 
were  formed,  and  these  were  filled  by  quartz  that  was  brought  in 
and  deposited  by  water. 

Then  passing  along  a  few  steps  to  the  west,  we  again  break  off 
a  fragment.  Here  the  rock  is  lighter  in  color,  because  of  the 
absence  of  the  graphite,  and  somewhat  sandy  in  appearance,  from 
the  larger  quantity  of  granular  quartz  in  it.  But  this  quartzose 
band  is  narrow,  and  then  we  again  find  the  normal  phyllite. 

At  about  forty  feet  from  the  east  end  on  the  north 
s^e  °^  *ke  track,  on  breaking  into  the  ledge,  we 
notice  a  peculiar  irregularity  of  the  surface  of  the 


MICA  SCHIST,  SHOWING  IRREGULARITIES  OF  SURFACE  DUE  TO  CONTAINED 

ANDALUSITES.    ORIGINAL,  5  INCHES  BY  4.     FROM  RAILROAD  CUT, 

NEAR  BLOOMINGDALE. 


GEOLOGY   OF   WORCESTER.  7 

laminae.  The  surface  looks  very  much  as  the  surface  of  one's  hand 
looks  when  a  sliver  has  been  forced  in  beneath  the  skin,  only  here 
the  apparent  slivers  are  very  abundant.  These  slivers  lie  in  the 
plane  of  the  laminae,  and  the  laminae  may  be  seen  at  the  edges 
wrapping  around  them.  At  the  edges  also  we  are  able  to  see  just 
what  the  slivers  really  are.  They  are  little  glassy  prisms  whose  ends 
are  frequently,  in  part,  covered  by  a  brassy  coating.  These  sliver- 
like  prisms,  giving  to  the  rock  this  peculiar  appearance,  are  probably 
crystals  of  andalusite,  a  mineral  frequently  found  in  this  rock  in 
other  places.  The  brassy  mineral  is  iron  pyrites,  which  is  also  fre- 
quently seen  in  this  rock.  This  latter  mineral  occurs  generally  not 
in  large  masses,  but  as  a  thin  coating  between  laminae,  and  on  the 
surfaces  of  fine  fissures,  even  making  up  fine  streaks  or  bands. 

For  a  hundred  feet  or  so  the  phyllite  continues  to  contain  these 
sliver-like  andalusite  crystals,  sometimes  more  abundantly  and  at 
times  less  so.  The  rock  then  becomes  the  normal  light  grey,  thinly 
fissile,  smooth,  soft  phyllite. 

Development  After  continuing  our  study  in  this  normal  phyllite  a 
of  a  secondary  short  distance,  when  we  are  about  two  hundred  feet 
from  the  bridge,  we  observe  an  unusual  appearance  on 
the  surface  of  the  ledge.  There  are  quite  regular  lines,  approximately 
parallel  and  horizontal,  crossing  the  face  of  the  ledge.  These  lines 
are  sometimes  an  inch  apart,  sometimes  two  or  even  three;  and 
above  these  lines  the  rock  frequently  projects  a  little,  as  if  some  of 
the  rock  just  below  the  line  had  fallen  away.  This  gives  to  the 
surface  an  unusually  ragged,  angular  appearance.  Examining  this 
appearance  more  carefully  we  find  that  it  has  a  deeper  meaning 
than  at  first  appears.  The  lines  are  cracks  or  fissures  cutting 
across  and  through  the  laminae,  not  between  them,  as  if  some 
one  with  a  thin,  sharp  knife  blade  had  cut  through  the  laminae 
as  he  might  cut  through  so  many  sheets  of  paper.  On  still  more 
careful  examination  we  see  that  the  cutting  was  not  done  so  nicely 
as  we  at  first  thought.  Each  little  lamina,  or,  better,  several  of 
these  laminae  are,  as  it  were,  stuck  together  and  bent  so 
that  their  edges  together  present  a  figure  having  this  shape,  ^v 
and  the  laminae  between  two  adjacent  fissures  are  arranged  (II 
in  a  succession  of  these  figures.  In  other 

words  the  upper  part  of  each  T\\S^\\\  lamina  is  bent 
towards  the  left,  and  the  lower  \\  \\  \\  \ \  part  towards  the 
right.  This  clearly  indicates  a  ^  v^  v^  ^  motion  between 
these  different  sections  by  which  the  upper  and 


8  GEOLOGY    OF   WORCESTER. 

lower  edges  of  the  laminae  in  a  section  are  bent  in  opposite 
directions.  Either  each  section  above  moved  more  to  the  left 
than  did  the  one  beneath,  or  each  section  beneath  moved  more 
to  the  right  than  did  the  one  above.  Possibly  this  structure  may 
result  from  simple  pressure  at  right  angles  to  the  planes  between 
these  sections,  by  which  a  yielding  at  right  angles  took  place, 
and  thus  the  laminae  have  come  to  have  an  undulating  form. 

This  structure  is  interesting  as  the  beginning  of  a  remarkable 
change.  Let  us  think  of  the  upper  edges  of  these  laminae  bent 
a  little  more  to  the  left,  and  the  lower  edges  a  little  more  to  the 
right;  then  again  the  upper  edges  still  more  to  the  left,  and  the 
lower  still  more  to  the  right;  it  is  evident  that  the  laminae  will 
approach  more  and  more  nearly  to  a  horizontal  position  until  they 
practically  reach  it,  and  the  laminae  of  one  unit  overlap  and  under- 
lap  the  laminae  of  the  adjacent  units  to  the  right  and  left.  When 
this  change  is  complete  the  segments  of  the  laminae  will  be  hori- 
zontal where  now  they  are  vertical.  This  is  a  clear  illustration 
of  how  the  laminae  of  a  rock  may  be  rotated,  even  to  a  position 
at  right  angles  to  their  former  position,  and  thus  the  capacity  of 
splitting  in  one  direction  obliterated  and  replaced  by  a  capacity 
of  splitting  in  a  new  direction.  This  may  be  called  the  develop- 
ment of  a  secondary  or  slaty  cleavage. 

From  these  observations  on  cleavage,  we  continue  our  study, 
still  going  to  the  west.  Nothing  new  attracts  our  attention  until 
we  are  within  a  hundred  feet  of  the  bridge.  Here  the  phyllite  is 
again  noticeably  dark  in  color,  with  the  smooth,  greasy  feel  charac- 
teristic of  graphite.  It  is  another  graphitic  band  in  the  phyllite, 
and  bands  like  this  are  not  infrequently  met  with  in  other  parts 
of  this  rock.  But  this  graphitic  band  is  only  a  few  feet  in  width, 
and  then  again  the  rock  is  the  normal  phyllite  until  we  reach  the 
bridge. 

On  the  west  side  of  the  bridge  we  immediately 
notice  a  marked  difference  in  the  appearance  of  the 
rock.  It  is  apparently  much  more  massive;  but  the 
fissile  structure  appears  when  we  break  a  fragment  from  the  ledge. 
The  ledge  is  cut  by  clearly  defined  joints  into  blocks  of  remarkable 
regularity.  This  rock  proves  to  be  quite  different  from  the  normal 
phyllite.  While  the  material  of  it  is  arranged  in  thin  laminae, 
the  rock  is  much  harder,  and  the  laminae  are  more  regular  and 
more  clearly  defined.  Under  the  magnifying  glass  it  is  seen  to 


DEVELOPMENT  OF  A  SECONDARY  STRUCTURE  BY  THE  ROTATION  OF  THE 
LAMINAE  IN  SECTIONS. 


GEOLOGY    OF    WORCESTER.  9 

be  made  up  of  bands  of  light  grey,  finely  granular  quartz,  alternat- 
ing with  bands  of  a  brownish  grey  color,  also  consisting  largely 
of  finely  granular  quartz.  These  bands  are  separated  by  thin, 
darker  sheets  of  dark  brownish  mica  in  fine  scales.  These  bands 
are  generally  as  clearly  defined  as  are  the  bands  in  an  agate,  and 
are  at  times  not  more  than  one  eighth,  one  sixteenth,  or  even  one 
thirty  second  of  an  inch  in  thickness.  While  these  bands  are 
generally  regular  and  extend  parallel  with  the  general  lamination 
of  the  rock,  now  and  then  one  or  more  is  crumpled  into  fine, 
closely  compressed  folds,  while  the  adjacent  bands  are  but  slightly 
disturbed.  In  this  are  also  found  bands  containing  flattened 
cylinders  of  light  grey,  slightly  reddish,  granular  quartz  inclosed 
by  micaceous  folia  folding  closely  about  them.  Farther  along  these 
flattened  cylinders  have  been  so  flattened,  as  it  were  rolled  out, 
as  to  be  almost  paper  thin;  and  then,  sometimes,  they  are  folded 
into  almost  innumerable  compressed  folds,  giving  to  the  rock  a 
coarsely  ribbed  appearance  on  the  lamination  surfaces.  This  rock, 
because  of  the  abundance  of  quartz  in  it,  may  be  called  a  micaceous 
quartzite.  This  continues  to  be  the  rock,  on  either  side  of  the 
tracks,  for  about  two  hundred  and  fifty  feet  from  the  bridge;  then 
the  quartzite  alternates  with  thin  bands  of  the  phyllite  for  one 
hundred  or  one  hundred  and  twenty-five  feet,  and  then  the  rock 
becomes  the  pure  phyllite,  practically  the  same  as  that  found  on 
the  east  side  of  the  bridge. 

Dike  of  apiite  While  making  this  study  of  the  quartzite,  breaking 
in  micaceous  off  fragments  constantly,  we  find  quite  a  different 
rock  in  the  midst  of  the  quartzite  on  the  north  side. 
It  is  white,  or  nearly  so,  in  color,  in  strong  contrast  with  the  border- 
ing rock.  It  is  possible  to  draw  a  line  clearly  marking  the  boundary 
of  the  white  rock.  It  is  also  divided  in  two  parts  by  a  thin  layer 
of  the  micaceous  quartzite.  The  whole  thickness  of  the  white 
rock  is  fifteen  feet.  On  closer  examination  this  rock  is  seen  to  be 
finely  granular  in  texture,  yet  divided  into  folia  as  thin,  and  almost 
as  regular,  as  those  of  the  bordering  quartzite.  In  the  finely 
grained  white  mass  we  may  distinguish  two  minerals — the  clear 
glassy  quartz  which,  now  and  then,  appears  in  rounded  particles 
many  times  the  size  of  the  surrounding  grains,  and  the  white 
porcelain-like,  finely  granular  feldspar.  These  make  up,  practi- 
cally, the  whole  of  the  rock.  In  addition  there  may  be  found, 
here  and  there,  a  little  pink  garnet.  Covering  the  surfaces  of 


10  GEOLOGY   OF   WORCESTER. 

the  folia,  and  seen  only  when  the  rock  is  broken  between  the  folia, 
are  minute  scales  of  muscovite  or  sericite  mica.  In  spite  of  the 
foliation  parallel  to  the  lamination  of  the  neighboring  rock,  we 
recognize  this  rock  as  a  foreigner,  not  really  belonging  with  the 
adjacent  rocks.  It  came  here  in  a  molten  state,  flowing  into  the 
fissure  which  it  now  occupies,  and  there  solidified,  forming  a  dike 
of  aplite,  a  variety  of  granite.  By  subsequent  pressure  and  crush- 
ing, the  separate  minerals  were  reduced  almost  to  powder,  and  the 
muscovito  scales  were  developed  on  surfaces  which  rubbed  against 
each  other,  thus  giving  to  the  rock  its  thinly  foliated  structure, 
weathering  From  this  aplite  dike  we  continue  our  study  of 
of  the  rock  of  the  facts  here  presented.  We  notice  quite  frequently 
that  there  is  a  narrow  part  or  band  of  the  ledge 
which  is  largely  concealed  by  a  steeply  sloping  bank  of  loose  rock 
material.  In  one  case  this  bank  may  reach  but  a  few  feet  up 
against  the  ledge;  in  another  case,  far  up  so  as  to  conceal  the  rock 
almost  to  the  top.  On  examining  this  fine  material,  we  find  it  to 
consist  largely  of  fine  scales,  evidently  derived  from  that  part  of 
the  ledge  which  it  conceals.  On  climbing  up  one  of  these  banks 
so  as  to  reach  the  ledge  above,  we  find  the  surface  of  the  ledge 
made  up  of  loosened,  scaly  particles  ready  to  fall  and  to  be  added 
to  the  bank  of  debris.  This  is  an  admirable  illustration  of  the 
way  in  which  such  rock  breaks  up  and  crumbles  under  the  action 
of  atmospheric  agents.  This  process  is  called  weathering. 

This  weathering  or  decay  is  partly  mechanical  and  partly 
chemical.  In  the  first  place  the  nearly  vertical  wall  of  rock  on 
either  side  is  made  up  of  many  uneven  surfaces,  and  contains 
innumerable  nooks  and  crannies.  The  ledge,  on  account  of  these 
irregularities,  presents  a  large  surface  for  action,  and  affords  con- 
venient lodgement  for  water,  snow  and  vegetation.  The  many 
joints,  cutting  the  ledge  in  different  directions,  afford  innumerable 
paths  by  which  the  water  may  enter  and  soak  through  the  rock. 
The  manyjtittle  streams,  dripping  from  the  walls  of  this  ledge,  even 
during  the  dryest  of  seasons,  show  very  plainly  that  the  water 
finds  these  capillary  channels,  and  makes  use  of  them.  Then  the 
frequent  jar  from  passing  trains  serves  to  open  the  joints  near 
the  surface,  and  loosen  the  blocks,  and  finally  throw  them  down 
the  talus.  The  texture  of  the  rock  also  favors  the  action  of 
weathering  agents.  Water  soaks  in  between  the  thin  laminae. 
The  soaking  waters  act  in  a  twofold  way.  During  the  cold  months, 


GEOLOGY   OF   WORCESTER.  11 

the  water  between  the  laminae,  or  in  joints,  or  cracks,  or  crevices 
of  any  kind,  freezes  and  expands,  and  thus  enlarges  these  spaces, 
and  breaks  the  rock  into  pieces.  In  this  way  many  blocks  are 
loosened  and  thrown  from  the  ledge.  The  laminae  are  forced 
apart  and  weakened,  and  are  ready  to  crumble. 

During  the  warm  months  of  the  year,  and  within  the  ledge 
where  frost  cannot  reach,  the  water  works  in  another,  yet  effective, 
way.  Dissolving  carbonic  acid  and  oxygen  from  the  air,  sulphur 
dioxide  from  the  smoke  of  the  passing  engines,  and  acids  from 
the  decaying  vegetation,  the  water  carries  these  into  the  rock  to 
act  upon  and  disintegrate  the  mineral  constituents,  and  thus  weaken 
and  break  up  the  rock.  But  as  the  water  carries  in  material,  it 
likewise  brings  out  products  of  change  and  decay.  The  lower 
surfaces  of  the  overhanging  rock  are  frequently  covered  by  an 
incrustation  left  by  the  evaporating  waters.  The  incrusting  sub- 
stances were  formed  in  the  rock,  dissolved  in  water,  brought  out  to 
the  surface,  and  there  deposited  as  an  incrustation  by  the  evaporation 
of  the  water.  The  thick  coating  of  iron  rust,  covering  the  beds 
of  the  brooks  by  the  side  of  the  tracks,  is  further  evidence  of  change 
within  the  rock.  That  iron  was,  formerly,  all  in  the  neighboring 
ledge. 

By  these  changes,  partly  mechanical  and  partly  chemical,  this 
rock  surface  is  being  disintegrated  and  reduced  to  fine  material, 
but  not  uniformly.  For  one  reason  or  another  one  band  is  acted 
on  much  more  rapidly  than  the  adjacent  bands  are,  and  there  is 
produced  a  recess  in  the  wall  and  a  high,  sloping  talus  of  fine 
debris. 

Having  noticed  these  interesting  facts,  we  continue  our  study 
of  the  rock  as  a  whole,  continuing  to  break  off  fragments  frequently 
for  examination.  We  constantly  find  the  light  grey  phyllite, 
having  finely  crinkled  surfaces,  and  breaking  into  thin  laminae 
whenever  struck  by  the  hammer. 

When  we  are  about  seven  hundred  feet  west  of 

Hornblende  . 

rock.  A  basic  the  bridge,  the  rock  suddenly  changes.  It  is  massive; 
eruptive  in  the  on  tjie  weathered  surface  it  is  of  a  light  rusty  color; 

cut. 

but,  within,  of  a  dark  greenish  grey.  Through  the 
magnifying  glass,  we  see  that  this  color  is  due  to  a  fibrous,  or 
bladed,  dark  green  mineral,  which  is  frequently  arranged  in  radiating 
masses,  one  half  inch  or  more  in  diameter.  This  mineral  we  recog- 
nize as  hornblende,  and  the  rock  is  a  massive  hornblende  schist. 


12  GEOLOGY   OF   WORCESTER. 

But  there  is  not,  so  far  as  we  can  see,  any  connection  between 
this  rock  and  the  neighboring  phyllite,  in  which  it  forms  a  band 
two  to  three  feet  wide.  It  is  in  the  phyllite,  but  not  of  it.  At 
some  time,  in  the  changes,  which  the  phyllite  has  been  through,  of 
folding,  of  elevation  and  depression,  of  crushing  and  breaking,  a 
fissure  was  formed,  where  now  we  see  the  hornblende  schist,  and 
into  this  fissure  flowed  some  basic,  molten  rock  which  solidified 
there,  making  a  dike.  By  subsequent  chemical  and  mechanical 
changes  in  the  rock  of  this  dike,  it  has  been  transformed  into 
this  massive,  hornblende  schist. 

We  then  continue  our  study  to  the  end  of  the 
Glacial  striae  cu^  four  hundred  and  twenty-five  feet  or  so  from 
of  the  ledge,  the  hornblende  schist,  but  find  only  the  normal  phyllite. 

If,  however,  we  go  up  on  to  the  surface  of  the  ledges, 
on  the  south  side  of  the  tracks,  we  may  see  something  of  interest 
to  us.  In  a  number  of  places  the  surface  of  the  ledge  is  covered 
with  grooves  and  scratches  parallel  to  one  another;  aside  from 
these  the  rock  surface  appears  smooth,  almost  polished.  By  means 
of  the  compass  we  take  the  direction  of  these,  and  find  them  point- 
ing five  degrees  east  of  south.  These  are  some  of  the  glacial 
scratches  so  frequently  found  on  the  surfaces  of  the  ledges  about 
Worcester,  and  show  us  the  direction  in  which  the  ice  of  the  Glacial 
Period  moved  over  this  region. 

We  have  thus  pointed  out  different  facts  that  may  be  observed 
in  passing  through  this  railroad  cut;  but  before  we  seek  for  the 
general  interpretation  of  these,  it  will  be  well  for  us  to  gather 
facts  from  another  typical  locality. 

Let  us  leave  the  electric  car  at  the  corner  of 
implantation  Plantation  and  Belmont  streets,  and  go  to  the 
street  on  the  north  through  the  former  street.  We  may  notice,  now 
"Coai'Mine*"  an<^  then,  by  the  side  of  the  street  small  outcrops  of 

the  phyllite  like  that  just  studied  at  the  railroad  cut. 
Then  all  trace  of  this  rock  disappears;  and,  crossing  the  first  small 
brook,  we  find  in  the  field,  just  over  the  wall,  on  the  left,  a  ledge 
of  rock  closely  resembling  the  micaceous  quartzite  just  west  of 
the  bridge  in  the  railroad  cut.  It  is  a  micaceous,  sandy,  granular 
quartzite.  Just  beyond,  in -the  gutter,  by  the  side  of  the  road, 
the  surface  of  the  ledge  is  smooth  and  covered  with  scratches  which 
we  recognize  as  glacial  scratches.  Taking  the  direction  of  these, 
we  find  them  pointing  thirty  degrees  to  the  east  of  south.  Com- 


WORCESTER  "COAL"  MINE.     MOUTH  OF  THE  OLD  SHAFT  is  AT  THE  FOOT  OF 
THE  CLIFF  IN  THE  FOREGROUND. 


GEOLOGY   OF   WORCESTER.  13 

paring  this  observation  with  that  made  at  the  railroad  cut,  we  note 
quite  a  variation.  This  is  due  to  some  local  cause  in  this  last  locality, 
as  we  do  not  generally  find  these  marks  pointing  so  far  to  the  east 
of  south.  We  also  take  the  direction  of  the  laminae  here,  and 
find  them  pointing  or  striking  thirty-  two  degrees  east  of  north. 
and  dipping  eighty  degrees  to  the  west.  If  we  follow  this  direction 
back  to  the  south,  we  find  that  these  beds  point  back  to  the  mica- 
ceous quartzite  of  the  railroad  cut.  There  is  every  indication. 
then,  that  these  different  outcrops  belong  to  one  and  the  same 
rock,  while  the  phyllite,  that  was  east  and  west  in  the  railroad 
cut,  does  not  appear.  A  glance  at  the  geological  map  will  make 
this  clear. 

As  we  proceed  in  this  study  our  eyes  must  con- 
n  bowl-     stantly  be  on  the  watch  for  facts.     A  stone  wall  may 


ders  by  the  often  afford  the  solution  of  some  difficulty,  or  it  may 
furnish  us  an  index  of  the  rocks  found  in  the  area 
around,  where  the  ledges  may  be  completely  concealed.  And  so, 
as  we  walk  along,  we  look  at  the  rocks  in  the  stone  wall  and  in 
the  gutters  by  the  roadside.  Soon  a  rock  attracts  our  attention 
because  of  its  peculiar  appearance.  It  resembles  the  granite  of 
the  neighboring  hill,  to  the  west,  in  color,  but  is  quite  different  in 
texture.  It  is  a  light  colored,  massive  rock,  the  larger  part  of 
which  is  finely  granular  in  texture,  and  more  or  less  rusty  in  color. 
In  the  midst  of  this  finely  granular  mass  are  distributed  crystals 
of  orthoclase  feldspar,  known  by  their  bright,  shining  surfaces,  which 
flash  as  the  specimen  is  turned  in  the  sunlight.  These  surfaces  are 
generally  longer  than  they  are  wide,  and  sometimes  show  the  sharp 
angles  of  definite  crystals.  In  the  granular  mass  are  distributed 
particles  of  quartz,  distinguished  from  the  feldspar  particles  by  their 
glassy  appearance  and  want  of  cleavage  surfaces.  These  quartz  par- 
ticles are  generally  slightly  rusty,  and  darker  in  color  than  are  the 
other  minerals.  Examining  one  after  another  of  these  particles 
under  the  magnifying  glass,  we  notice  a  certain  regularity  of  shape 
in  one.  It  may  be  well  for  us  also  to  examine  the  cavities  left  by 
some  of  these  particles  as  the  rock  broke  under  the  hammer.  We 
finally  see  a  quartz  particle  having  a  definite,  regular  shape.  It  is  a 
six-sided  pyramid.  We  may  possibly  be  fortunate  enough  to  find  a 
particle  showing  a  six-sided  pyramid  at  either  end.  The  quartz  in 
this  rock  is,  now  and  then,  in  the  form  of  a  regular  crystal,  a  double, 
six-sided  pyramid.  A  specimen  of  this  rock,  rough  and  uninterest- 


14  GEOLOGY   OF  WORCESTER. 

ing  as  it  may  at  first  seem,  should  be  preserved,  for  it  is  an  uncom- 
mon rock  here  in  Worcester.  The  abundance  of  the  fragments  in 
the  walls  indicates  that  it  occurs  hi  the  ledge  in  this  immediate 
vicinity.  So  far  as  is  known,  the  part  of  the  rock-floor  consisting 
of  this  rock  is  entirely  covered  by  the  glacial  material.  Because  of 
the  crystals  imbedded  in  the  finely  granular  mass,  this  rock  may  be 
called  a  granite  porphyry.  It  is  probably  closely  related  to  the 
granite  of  Millstone  Hill,  of  which  more  will  be  said  later  on. 
Approaching  ^u*  we  mus^  n°t  forget  that  we  are  on  our  way 
the »coai  to  the  coal  mine,  located  on  the  Swan  Farm;  and 
so,  just  west  of  the  southern  end  of  Wigwam  Hill, 
we  turn  into  the  fields  on  the  left.  Here,  also,  the  stone  wall  will 
reward  our  close  observation.  Bowlders  may  be  seen  here,  con- 
sisting of  layers  which  have  been  folded  in  an  exceedingly  complex 
manner.  This  folding  is  beautifully  brought  out  by  the  weathering, 
which  has  given  a  reddish  tint  to  some  of  the  layers.  Going  to 
the  west,  we  ascend  the  low  hill  and  turn  to  the  north.  The  stone 
wall  tells  us  of  a  change  in  the  ledge.  The  wall  is  now  made  up 
entirely  of  angular,  flat,  thin  fragments  of  a  light  grey,  finch'' 
grained  quartzite,  having  many  thin,  white  quartz  veins,  making 
almost  a  network  through  the  rock.  The  abundance  of  these  frag- 
ments leaves  no  doubt  but  that  the  ledge  beneath  is  of  the  same 
kind.  One  thing  is  specially  noticeable  that,  at  some  period  in  its 
history,  this  rock  was  shattered,  producing  innumerable  little  fissures, 
which  were  afterwards  filled  with  quartz,  constituting  the  quartz 
veins.  These  veins  now  stand  out  in  relief  because  they  are  more 
resisting  to  the  agents  of  the  atmosphere  than  is  the  rock  itself. 
Continuing  to  the  north,  along  the  crest  of  the  slope,  when  almost 
directly  west  from  the  barn  of  the  Swan  Farm,  we  come  to  an 
overhanging  cliff;  beneath  this  is  the  mouth  of  the  coal  mine  shaft. 
This  is  a  point  of  special  geological  interest,  and 

Coal  Mine.          •     i          i_  •  •  •      *  t 

it  has  been  visited  by  many  geologists.  We  cannot 
make  too  careful  study  here,  for  in  this  place  is,  in  a  certain  sense, 
the  key  to  the  geology  of  Worcester.  At  various  points  around,  the 
ledge  appears  at  the  surface.  It  is  a  very  dark  grey,  almost  black, 
phyllite  or  graphitic  schist,  presenting,  on  cleavage  surfaces,  a 
glimmering  lustre.  It  generally  breaks  in  thin,  regular  slabs,  or 
in  long,  narrow,  thin  masses.  It  crocks  the  hands  badly;  and  in 
a  short  time  the  hammer  handle  looks  as  if  it  had  received  a  coating 
of  stove-polish.  On  the  cross-section  the  schist  is  of  a  dull  black 


GEOLOGY   OF   WORCESTER.  15 

color.  We  take  the  dip  and  strike,  and  find  considerable  variation. 
In  some  places  the  laminae  point  east  and  west,  and  in  others 
some  degrees  north  of  east,  but  always  far  more  to  the  east  than 
in  any  other  place  where  we  have  thus  far  made  observations. 
This  clearly  indicates  a  decided  warping  or  breaking  in  the  laminae 
of  the  rocks  between  here  and  Wigwam  Hill,  just  across  Plantation 
street.  The  rocks  of  this  area  have  evidently  been  subjected  to 
great  disturbances,  by  which  they  have  been  warped,  folded,  crum- 
pled, even  broken.  This  will  become  more  evident  as  we  proceed 
in  our  study. 

iron  pyrites  ^e^  us  now  examme  the  rock  of  the  cliff  overhang- 

in  rock  at  Coal     ing  the  mouth  of  the  old  mine.     The  laminae  have  a 

general  slope  or  dip  to  the  north  or  northwest.  There 
is  quite  a  variation  in  color.  There  are  shades  of  yellow  and  brown 
mixed  with  light  and  dark  grey,  while  here  and  there  appear 
patches  of  green  vegetation.  These  colors,  excepting  the  green, 
are  due  to  iron  rust  and  other  substances  resulting  from  the  oxidation 
of  the  iron  pyrites  which  is  distributed  quite  abundantly  through 
the  rock.  This  mineral  is  sometimes  found  here  as  a  thin,  light 
brassy  coating  between  the  laminae,  and  sometimes  in  thin  veins 
and  crystalline  masses.  It  is  oxidized  by  oxygen  carried  into  the 
rock  in  solution  in  water,  and  forms  either  a  sulphate  of  iron  and 
sulphuric  acid,  or  an  acid  sulphate.  These  in  turn,  especially  when 
in  solution,  act  on  other  minerals,  and  produce  a  variety  of  sub- 
stances which  appear  on  the  surface  of  the  cliff.  From  this  source 
comes  the  yellowish  coating  of  iron  rust,  appearing  at  the  bottom 
of  the  pool  beneath  the  cliff.  But  how  does  it  happen  that  there 
is  iron  pyrites  distributed  through  the  rock?  The  rock  is  highly 
charged  with  carbonaceous  material,  which  gives  to  it  a  black 
color.  This  carbonaceous  material,  or  that  from  which  this  was 
formed,  has  acted  as  a  reducing  agent  upon  iron  sulphate,  taking 
oxygen  from  it,  and  has  changed  the  sulphate  into  iron  pyrites. 
This  iron  sulphate,  in  solution  in  water,  was  carried  into  the  mud 
while  the  rock  was  forming,  and  the  water  percolated  through  the 
crevices  and  fissures,  and  soaked  into  capillary  spaces,  and  there 
the  change  from  sulphate  to  iron  pyrites  took  place. 

Let  us  now  break  into  the  rock  of  the  cliff.     It 
thTeoa^iine     *s  a^  a  dark  grey  or  black  carbonaceous  schist  or 

phyllite  like  that  of  the  adjacent  outcrops;  but  as 
the  laminae  slant  up  to  the  southwest,  lower  beds  appear  in 


16  GKOLOHY    OF  WORCESTER.. 

that  direction,  and  these  may  best  be  studied  at  the  western 
end  of  the  cliff.  Beneath  the  upper  black  schist  is  the  bed  of 
coal,  or,  better,  graphite,  which  is  probably  the  bed  that  was  once 
mined,  as  this  bed  slopes  down  so  as  to  pass  beneath  the  overhang- 
ing cliff  where  is  now  the  mouth  of  the  shaft.  This  bed  is  some  five 
or  six  feet  thick,  somewhat  thicker  than  was  the  bed  formerly  mined, 
as  it  is  described  by  Dr.  Hitchcock  and  others.  Where  the  hammer 
strikes,  the  substance  of  the  rock  appears  of  a  dark  grey  color, 
with  metallic  lustre,  and  soft,  smooth,  greasy  feel.  This  substance 
has  the  characteristics  of  graphite.  But  to  be  more  careful  before 
drawing  our  conclusions,  we  break  off  some  fragments.  The  rock 
substance  in  part  consists  of  white  glassy  quartz,  more  or  less 
granular,  distributed  irregularly  through  the  mass,  while  the  greater 
part  is  of  thin  scaly  or  flaky  particles,  the  larger  ones  a  half  to 
three  fourths  of  an  inch  broad  and  long,  arranged  generally  par- 
allel to  the  lamination.  These  flakes  have  the  dark  grey  color, 
metallic  lustre,  smooth  greasy  feel  of  graphite,  and  crock  the  hands 
and  other  objects,  giving  to  them  a  coating  having  the  same  char- 
acteristics. Here  also  our  hammer  handle,  in  a  short  time,  appears 
as  if  it  had  been  covered  with  a  substantial  coat  of  stove-polish. 
This  substance  certainly  has  the  outward  properties  of  graphite, 
but  as  there  has  been  some  question  whether  it  is  really  graphite, 
or  a  variety  of  anthracite  coal,  it  was  subjected  to  the  final  test. 
If  we  can  prepare  graphitic  acid  from  it,  we  may  conclude  that 
it  is  graphite;  if  not,  then  we  must  conclude  that  it  is  only  a 
variety  of  anthracite  coal. 

Some  of  the  material  from  the  coal  mine,  in  the 

Glfromticoaikl  form  of  a  ^  Powder>  was  treated  according  to  the 
Mine  graphite,  directions  for  preparing  graphitic  acid  from  graphite; 
and,  at  the  same  time,  powdered  graphite  from 
Ceylon  was  treated  in  the  same  way.  From  each  there  was  obtained 
a  yellowish  colored  substance  agreeing  in  all  respects  with  the 
description  of  graphitic  acid.  In  fact  it  was  easier  to  oxidize  the 
coal-mine  substance  to  graphitic  acid  than  it  was  to  oxidize  the 
Ceylon  graphite.  From  this  we  conclude  that  the  substance  at 
the  coal  mine  is,  in  large  part,  graphite. 

Graphite  To  return  to  the  specimens  we  were  examining — 

breccia.         we  described  the  graphite  as  being  in  flakes.     These 

flakes  are  bordered  by  irregular,  angular  edges,  and  are  imbedded 

in  finer  material,  also  made  up  of  finer,  irregular,  angular  flakes 


GRAPHITE  BRECCIA.    IRREGULAR  DARK  PATCHES  ARE  GRAPHITE  FLAKES. 


GEOLOGY   OF   WORCESTER.  17 

not  parallel  to  each  other.  This  structure  becomes  more  clear  in 
more  impure  specimens,  where  the  angular  flakes  and  their  bedding 
are  very  evident.  Mixed  with  these  is  a  considerable  quantity  of 
glassy  quartz  between  the  flakes.  From  this  study  it  is  evident 
that  this  graphite  bed  is  simply  a  mass  of  angular,  flaky  fragments. 
But  if  the  graphite  is  now  in  this  condition,  it  must,  at  some  period 
in  its  history,  have  been  completely  shattered,  and  this  period 
must  have  been  since  the  carbonaceous  material  became  graphite; 
and  then  these  fragments  must  have  been  pressed  together  and 
quartz  deposited  between  them  so  as  to  bring  this  substance  into  its 
present  condition.  This  graphite  bed  has  all  the  characteristics  of 
a  graphite  breccia. 

Graphite  phyi-  Under  the  graphite  breccia  band  is  a  band  or  layer 
thegnvhite  of  hard  phyllite  of  a  black  color,  and  abounding  in 

breccia.  graphite.  This  band  has  an  irregularly  broken,  lami- 
nated structure  due  to  the  fact  that  the  material  of  the  rock 
is  not  arranged  in  regular  laminae,  but  in  flattened,  irregular, 
lengthened  lenses  varying  in  size.  Many  are  from  five  to  seven 
inches  in  length,  by  three  to  four  inches  wide,  by  an  inch  and  a 
half  thick.  These  are  so  placed  in  the  ledge  that  their  longest  diame- 
ters are  parallel,  and  when  the  surface  of  the  ledge  is  at  right  angles 
to  these,  the  face  of  the  ledge  consists  of  the  sharp  ends  of  these 
lenses,  giving  a  ragged,  hackly  surface.  The  surface  of  each  irregu- 
lar lens  is  shining,  smooth,  and  striated,  presenting  the  appearance 
that  is  called  slickensides,  and  shows  the  lustre  and  color  of  graphite; 
while  the  interior  is  of  a  dull  black  color.  Just  how  much  of  the 
substance  of  these  is  graphite  is  difficult  to  say,  but  judging  from 
appearance  a  considerable  part  must  be.  However,  other  fragments 
when  heated  and  then  boiled  with  hydrochloric  acid,  lose  most  of 
the  black  powder,  so  part  of  it  must  be  still  coal.  This  material 
gives  evidence  of  the  same  shattering  that  was  noticed  in  the 
graphite  layer,  but  the  fragments  are  coarser,  and  they  give  evidence 
of  a  crunching  of  the  rock  by  some  outside  force,  by  which  they 
have  become  rounded  and  flattened,  and  their  surfaces  polished 
and  striated  from  rubbing  one  against  another.  The  meaning  of 
all  this  will  appear  in  another  connection. 

Fibrous  But  there  is  here  still  more  of  interest  for  us.     On 

prochiorite.      breaking  into   the   ledge  overhanging  the  mine,  we, 

now  and  then,  notice  some  very  fine  fibres  in  layers  or  small  masses. 

The  same  appears  in  the  other  outcrops  near  by.     These  fibres 

3 


18  GEOLOGY   OF   WORCESTER. 

sometimes  occur  in  seams  or  veins  an  inch  or  two  in  thickness,  the 
fibres  running  across  from  side  to  side  of  the  vein.  This  mineral 
also  occurs  as  a  very  thin  coating,  mingled  with  glassy,  fibrous 
quartz,  on  striated  slickensides,  and  also  in  irregular  masses  within 
the  graphite  breccia.  This  mineral  is  of  a  light  green  color,  when 
not  decomposed,  but  is  frequently  of  a  rusty  tint  because  some  of 
the  iron  in  it  has  rusted  out.  The  mineral  is  also  characterized 
by  a  beautiful  silky  lustre.  It  is  not  strange  that  this  mineral 
has  been  called  asbestos,  so  closely  does  it  resemble  that  mineral, 
but  it  has  not  the  composition  of  the  asbestos  of  the  amphibole 
group,  nor  that  of  chrysotile,  the  fibrous  serpentine.  It  was  ana- 
lyzed by  Mr.  L.  G.  Eakins  in  the  laboratory  of  the  U.  S.  Geological 
Survey  at  Washington,  and  its  composition  shows  that  it  is  a  fibrous 
form  of  prochlorite. 

On  one  of  his  visits  to  Worcester,  Louis  Agassiz 
nehe  was  taken  to  this  coal  mine,  as  a  point  of  special 
geological  interest.  "Where  are  the  fossils?"  he 
asked.  On  being  informed  that  none  had  ever  been  found,  he 
stated  that  they  were  there  and  would  be  found,  if  only  careful 
search  were  made.  On  the  6th  day  of  August,  1883,  the  first 
fossil  plant  was  found.1  The  surface  of  one  side  of  it  is  represented 
in  the  frontispiece,  and  the  other  surface  is  represented  in  the  plate 
opposite.  These  represent  the  flattened  trunk  of  a  tree.  Buried 
beneath  many  feet  of  earth,  this  tree-trunk  was  compressed  or 
flattened  so  that  now  it  is  not  cylindrical  in  shape.  Again  in  the 
fall  of  1899,  another  fossil,  represented  in  the  accompanying  plate, 
was  found,  which,  strange  as  if  may  seem,  exactly  fits  that  found 
sixteen  years  before,  and  is  the  cast  of  one  surface  of  that  tree- 
trunk,  the  cast  of  the  other  side  having  been  obtained  in  1883, 
with  the  trunk  itself.  Mr.  Calvin  H.  Andrews,  teacher  in  the 
South  High  School,  states  that  he,  while  visiting  the  coal  mine 
during  his  course  at  the  Polytechnic  Institute,  also  found  a  fossil 
plant.  This,  unfortunately,  was  lost.  There  can  be  no  doubt  but 
that  fossils  occur  at  this  locality,  in  spite  of  the  great  change  to 
which  the  rock  has  been  subjected,  and  those  who  seek  long  and 
patiently  may  be  rewarded.  If,  in  breaking  the  rock  at  the  coal 
mine,  any  strange  or  regular  markings  are  observed,  the  specimens, 
showing  these  should  be  carefully  preserved  because  of  their  scien- 
tific value  and  interest.  These  fossils  tell  us  an  interesting  story  in 
regard  to  the  origin  of  this  graphite  bed.  The  carbon  of  it  was  once 

i  "  American  Journal  of  Science,"  Vol.  XXIX.,  1886,  p.  157. 


LEPIDODENDRON   ACUMINATUM    (LESQUEKEUX).    ONE-THIRD   NATURAL 

SIZE.    REVERSE  SIDE  OF  FOSSIL  SHOWN  IN  FRONTISPIECE.    FOUND 

AT  WORCESTER  "COAL"  MINE,  1883.    PHOTOGRAPH  BY 

JOHN  M.  BLAKE. 


LEPIDODENDRON     ACUMINATUM.     ONE-THIRD     NATURAL     SIZE.     FOUND 
AT  WORCESTER  "COAL"  MINE,  1899.    PHOTOGRAPH  BY  E.  B.  LUCE. 


GEOLOGY   OF   WORCESTER.  19 

in  vegetable  substances  of  plants  and  trees  growing  upon  the  earth ; 
these  substances  became  imbedded  in  mud,  and  were  transformed 
probably  first  into  peat,  then  to  coal,  and  last,  in  large  part,  into 
graphite. 

But  this  graphite  bed  with  its  fossils  tells  us  more. 
of  thTphyime.  As  ft  occurs  m  the  phyllite,  it  tells  us  that  this  rock 
has  not  always  been  as  it  now  is.  It  tells  us  that  the 
rock  has  been  made  out  of  that  material  in  which  vegetable  matter 
would  most  likely  be  buried — mud  or  beds  of  clay.  And  what 
is  true  of  the  phyllite  at  the  coal  mine,  is  also  true  of  that  at  the  rail- 
road cut,  already  studied,  for  the  latter  contains  graphitic  bands. 
In  other  parts  of  this  phyllite,  both  in  Worcester  and  in  other  towns, 
are  found  bands  more  or  less  graphitic,  so  that  in  spite  of  the  slight 
variations  in  outward  appearance  and  crystallization  and  minerals 
contained,  we  may  conclude  that  the  phyllite  was  once  in  the  form  of 
beds  of  mud  or  clay,  and  during  a  part  of  its  history  supported  a  luxu- 
riant vegetation.  The  remains  of  these  plants  became  buried  in  the 
mud  of  marshes,  and  have  been  transformed  into  the  graphite  of 
these  graphitic  bands. 

Metamoi--  But  we   may   justly  ask   by   what   change,   and 

mat^n'of  the      ^v  what  agents>   be^s  of   mud  or  clay  have  been 

phyllite.  transformed  into  this  phyllite  or  fine  mica  schist. 
We  may  partly  answer  this  question  by  studying  specimens  of 
phyllite  from  different  localities.  Some  of  these  show  but  a 
glimmering  surface,  even  under  the  magnifying  glass;  while  others 
show  small  scales  of  mica  as  the  essential  part  of  the  laminae; 
while  all  under  the  large  microscope,  are  seen  to  be  made  up, 
in  very  large  part,  of  mica  scales  and  quartz  grains.  Further 
study  shows  that  these  mica  scales  were  not  deposited  by  water 
as  a  part  of  the  mud,  for  they  give  no  evidence  of  having 
been  washed  about  and  broken  by  contact  with  other  particles. 
They  have  the  appearance  of  having  been  formed  in  the  rock 
where  they  are.  But  they  are  crystals,  more  or  less  nearly  perfect, 
and  constitute  the  great  mass  of  the  rock.  The  change,  then, 
that  has  taken  place  must  have  been,  in  part  at  least,  a  crystalli- 
zation of  the  former  muds  or  clays.  But  a  mere  crystallization 
implies  that  the  substances,  before  and  after  crystallization,  are 
identical  except  in  form.  Not  so,  however,  with  the  beds  of  un- 
changed mud  or  clay  and  the  crystallized  rock  material  formed 
from  them.  The  former  is  not  a  single  substance,  but  a  mixture 


20  GEOLOGY   OF   WORCESTER. 

of  various  substances  that  happen  to  be  carried  by  currents  of 
water  and  deposited  because  those  currents  are  no  longer  able,  for 
one  reason  or  another,  to  carry  the  particles  farther.  In  such 
deposits,  in  a  region  like  this,  will  be  the  kaolin  from  the  decayed 
feldspar  of  the  granite;  also  the  ground  up  feldspar  which  escaped 
decay,  but  has  been  reduced  to  powder  by  the  wear  to  which  the 
granite  pebbles  have  been  subjected  in  the  brooks  and  rivers; 
and  mica  and  very  fine  quartz,  also  from  the  neighboring  crystalline 
rocks.  With  such  a  complex  mixture  as  this,  something  more  than 
mere  crystallization  must  take  place  to  transform  these  sediments 
into  a  rock  in  which  one  mineral  predominates.  There  must  take 
place  a  chemical  action  by  which  the  elements  of  the  old  sediments 
are  rearranged  into  the  minerals  of  the  schist.  This  is  the  change 
that  has  occurred  in  the  beds  of  mud  and  clay,  the  elements  in  the 
substances  of  these  have  been  rearranged  into  new  substances, 
with  possibly  the  exception  of  the  very  fine  sand,  and  these  sub- 
stances, including  the  fine  sand,  have  been  crystallized  into  the 
crystalline  particles  and  grains  and  definite  crystals  found  in  the 
phyllite  and  mica  schist. 

Such  being  the  change  that  the  mud  or  clays  have 
A  *"     undergone  in  becoming  this  phyllite,  we  may  ask  what 


agents  have  taken  part  in  this  change,  for  this  mixture 
might  remain  indefinitely  under  ordinary  conditions  without  ever 
forming  any  new  minerals,  or  the  old  ones  ever  becoming  crystal- 

lized.    Pressure  has  undoubtedly  been   an  element 

in  this  change.  Much  phyllite  has  been  removed  by 
erosion,  so  that  what  is  now  at  the  surface  was  once  deeply  buried 
beneath  the  overlying  mass,  and  hence  subjected  to  the  pressure 
of  that  weight.  Moreover,  this  rock  has  been  tremendously  com- 
pressed and  folded  and  crumpled  in  an  exceedingly  complex  manner. 
During  this  folding  and  compression  the  mixed  sediments  were 
subjected  to  great  pressure  resulting  in  motion  between  the  particles, 
as  well  as  in  the  motion  of  large  masses.  Pressure,  especially 
when  accompanied  by  motion,  is  sufficient  to  induce  chemical 
action  between  substances  capable  of  mutual  action,  especially 
when  this  action  is  accompanied  by  contraction,  that  is,  when 
the  new  substances  are  more  dense  or  occupy  less  space  than  the 
old  substances  did. 

But  the  folding  and  crumpling  of  strata  must  have 

produced  friction.     Stratum  rubbed  against  stratum, 


GEOLOGY   OF   WORCESTER.  21 

and  particle  against  particle,  and  thus  generated  heat  within  the 
very  rock  substance.  This  heat  of  friction  must  have  been  more 
or  less  increased  by  heat  from  within  the  earth,  the  increase  depend- 
ing on  the  depth  at  which  this  rock  material  was  beneath  the  surface. 
Possibly  also  the  heat  of  this  rock  mass  was  increased  by  that  derived 
from  molten  rock  masses  that  rose  from  beneath  into  the  midst 
of  these  sediments  or  near  to  them.  Heat,  also,  of  itself  is  capable 
of  inducing  chemical  action,  and  would  be  sufficient  to  produce 
changes  in  these  beds  of  clay,  were  other  conditions  normal. 

Moreover  these  sediments  would,  from  the  manner 
in  which  they  were  deposited  and  from  their  position, 
be  moist.  The  solvent  power  of  water  is  vastly  increased  by 
pressure  and  heat,  so  that  it  dissolves  mineral  substances  not  soluble 
under  ordinary  conditions.  Substances  in  solution  much  more 
readily  act  chemically  because  of  the  greater  ease  of  motion  and 
closer  contact.  Thus  we  see  that  the  moisture  in  these  sediments 
helped  in  the  formation  of  new  substances  out  of  the  old. 

uniformity  Under  the  conditions  through  which  those  beds 

m  position  of      of  c}av  nave  passed,  the  various  mineral  substances 

and  reason  were  subjected  to  the  combined  influence  of  these  vari- 
therefor.  ous  agen^s  of  change,  and  being  capable  of  acting  on 
each  other,  formed  new  substances.  The  new  substance  formed 
in  greatest  abundance  was  mica.  But  as  mica  forms,  it  tends  to 
assume  the  shape  of  little,  thin,  scale-like  crystals.  These  little 
scales,  more  or  less  nearly  perfect  crystals,  as  formed,  assumed  a 
certain  parallelism  in  their  position,  giving  the  cleavage  that  is 
so  marked  in  the  schist.  It  is  reasonable,  then,  that  we  should  ask 
why  these  mica  scales  so  arranged  themselves  as  this  new  substance 
was  formed.  Let  us  think  of  a  rock  mass  subjected  to  pressure 
from  all  directions,  and  the  pressure  from  one  direction  equal  to 
that  from  every  other  direction.  The  simple  result  of  such  pressure 
would  be  compression,  the  particles  being  forced  nearer  and  nearer 
together.  This  pressure  from  all  directions  may  be  resolved  into 
three  pressures,  at  right  angles  to  each  other — an  up  and  down 
pressure,  an  east  and  west  pressure,  and  a  north  and  south  pres- 
sure. Now  let  us  think  of  unequal  pressures — one  greater  than 
either  of  the  other  two,  and  these  other  two  equal.  Let  us  also 
think  of  the  greater  pressure  exceeding  the  others  by  more  than 
the  strength  of  the  rock  under  consideration.  Let  a  rock  mass 
then  be  subjected  to  these  three  pressures  at  right  angles  to 


22  GEOLOGY   OF  WORCESTER. 

each  other.  Now,  instead  of  simple  compression,  as  when  the 
pressures  were  equal,  there  will  be  a  yielding  in  the  plane  of  the 
smaller  pressures,  and  at  right  angles  to  the  larger  pressure.  Let 
us  now  think  of  mica  scales  being  built  up  in  this  yielding  mass. 
They  will  be  built  up  in  the  direction  of  the  yielding, — not  against 
the  greater  pressure,  but  in  the  direction  of  the  smaller  pressure. 
As  a  result  of  this  arrangement,  the  mica  scales  lie  in  the  plane 
of  the  smaller  pressures  and  at  right  angles  to  the  greater  pres- 
sure. As  a  result  of  this  arrangement  of  the  mica  scales  there 
will  be  a  rock  or  slaty  cleavage.  Other  minerals  like  chiastolite 
and  staurolite,  formed  at  the  same  time  and  capable  of  growing 
in  one  or  two  directions  more  easily  than  in  the  third,  also  ar- 
range themselves  in  the  plane  of  less  pressure;  while  other  minerals, 
like  garnet,  only  capable  of  growing  in  all  directions  with  equal 
ease,  are  not  affected  by  the  greater  pressure  and  are  found  imbed- 
ded in  the  schist  without  order  or  special  arrangement.  The 
cleavage  and  the  arrangement  of  the  mica  scales,  and  of  other 
minerals  within  the  phyllite,  are  due,  then,  to  the  greater  pressure 
that  was  exerted  in  one  direction  during  the  crystallization  of 
these  minerals.  As  the  mineral  matter  of  the  rock  was  undergoing 
this  change  and  crystallization,  the  beds  of  vegetable  matter 
buried  in  the  rock  underwent  a  like  change,  and  now  appear  largely 
in  the  form  of  graphite. 

original  From  these  last  considerations  it  is  evident  that 

layers  of  the  the  lamination,  so  characteristic  of  the  phyllite,  has 
no  necessary  connection  with  the  original  layer  struct- 
ure of  the  mud  or  clays.  It  becomes  an  interesting  problem 
for  us  to  determine,  if  possible,  something  in  regard  to  these 
original  layers,  that  we  may  the  better  understand  the  changes 
in  position  that  may  have  taken  place.  All  that  we  have  been 
able  to  find  that  gives  us  indications  of  the  original  rock  layers 
is  the  variation  in  the  phyllite  noticed  in  the  descriptions  of  the 
different  phases  or  bands  at  the  railroad  cut  near  Bloomingdale. 
The  graphite  bands  and  the  chiastolite  band,  alternating  with  bands 
of  the  normal  phyllite,  indicate  layers  or  strata  varying  in  compo- 
sition, hence  probably  indicate  the  layers  or  strata  in  the  original 
sediments.  As  these  bands  are  now  parallel  with  the  lamination 
of  the  phyllite,  the  indications,  so  far  as  they  go,  show  that  the 
original  strata,  at  least  in  this  place,  have  been  upturned  or  folded 
so  as  now  to  stand  almost  vertically.  In  no  other  place  do  we 


RAILROAD  CUT  AT  THE  SUMMIT  ON  THE  B.  &  M.  R.  R. 


GEOLOGY    OF   WORC'ESTER.  23 

find  so  good  opportunity  to  trace  out  the  original  structure  of 
the  rock;  so,  reasoning  from  this,  until  there  is  something  found 
to  the  contrary,  we  may  conclude,  in  general,  that  the  lamination 
is  parallel  with  the  original  structure  of  the  rock. 

Though  we  have  now  traced  the  phyllite  back 
€  through  chemical  and  dynamic  changes  to  the  original 
sedimentary  strata,  the  study  of  this  rock  is  not 
complete.  There  are  still  localities  of  great  interest  which  pre- 
sent facts  not  yet  pointed  out,  or  not  so  well  illustrated  elsewhere. 
Such  a  locality  is  the  railroad  cutting  near  the  Summit  station 
on  the  Boston  and  Maine  railroad.  Let  us  begin  in  our  study 
at  the  end  nearer  the  city.  The  rock  here  is  the  phyllite  not  to 
be  distinguished  from  that  first  described,  occurring  in  Court  Hill. 
We  take  the  dip  and  strike,  and  find  the  laminae  pointing  nearly 
north,  and  dipping  thirty  degrees  to  the  west.  We  do  not  progress 
far  in  our  observations  before  we  notice  that  the  ledge  is  cut  by 
many  quartz  veins.  These  are  made  up  of  milky 
or  glassy  quartz,  arid  vary  in  thickness  up  to  eigh- 
teen inches.  There  is  no  regularity  in  the  position 
of  these  veins.  It  is  impossible,  generally,  to  trace  one  of  these 
veins  more  than  a  few  feet,  for  they  do  not  extend  regularly  and 
continuously,  as  veins  usually  do;  but  they  have  been  involved 
in  the  folding  and  crumpling  to  which  the  rock  has  been  subjected. 
As  a  result  of  this,  in  addition  to  the  complex  folding  which  they 
present,  they  have  been  compressed  in  places  so  that  they  consist  of 
bulging  masses ;  at  other  places  they  have  been  squeezed  out  to  mere 
filaments  or  streamers.  To  such  an  extent  have  these  veins  been 
modified,  that  rarely  can  the  parts  of  the  same  veins  be  matched. 
In  fact,  the  folding,  and  crumpling,  and  squeezing  together,  and 
pinching  out  and  stretching,  have  gone  so  far  in  places  as  to 
produce  a  perfect  medley  of  schist  and  quartz  veins.  These  facts 
have  their  meaning  and  contribute  to  the  history  of  the  phyllite. 
The  quartz  veins  tell  us  of  the  extensive  fracturing  of  the  rock, 
by  which  both  large  and  small  fissures  were  formed.  Waters  then 
carried  in  quartz,  filling  the  fissures  and  mending  the  fractures. 
At  a  later  time  this  rock  was  subjected  to  tremendous  pressure 
exerted  against  the  edges  of  the  laminae.  So  great  was  this  pres- 
sure and  such  the  condition  of  the  phyllite  and  quartz  of  the  veins 
that  they  were  no  more  resisting  than  is  clay  in  the  potter's  hand, 
and  both  were  crumpled  into  innumerable  folds  and  faults. 


24  GEOLOGY   OF  WORCESTER. 

Double  car-  While  studying  the  quartz  veins  our  attention  is 

bonate  of  iron  attracted  by  the  rusty  appearance  presented  by  a  part 
um'  of  a  vein.  On  closer  observation  we  see  that  the  rusty 
part  is  not  glassy  quartz,  but  is  soft  and  easily  scratched  by  the 
knife  blade.  The  mineral  has  a  cleavage,  and  tends  to  break  into 
rhombohedra.  A  drop  of  hydrochloric  acid  tells  us,  by  the  efferves- 
cing, that  the  mineral  is  a  carbonate.  The  iron  rust  everywhere 
coating  it  tells  that  iron  must  be  in  this  mineral.  It  is  in  reality  a 
double  carbonate  of  iron  and  calcium.  We  may  think  of  it  as 
calcite  in  which  a  considerable  part  of  the  calcium  is  replaced  by 
iron,  and  the  mineral  is  called  ankerite.  This  mineral  occurs 
quite  generally  with  the  quartz,  constituting  a  part  of  the  vein. 
It  too  must  have  been  brought  in  and  deposited  by  water,  either 
when  the  quartz  was  deposited,  or  later  in  cavities  occupied  by 
some  other  mineral,  which  was  deposited  with  the  quartz  and  was 
subsequently  removed.  But  this  calcite  is  by  no  means  fixed  in 
its  position.  It  is  soluble  in  water  containing  carbon  dioxide  or 
carbonic  acid  gas.  It  may  be  taken  up  by  percolating  waters, 
and  carried  until,  because  of  the  escape  of  the  carbonic  acid  gas 
from  the  water,  the  mineral  is  no  longer  held  in  solution  and  so  is 
deposited.  Now  and  then  we  may  find  a  considerable  vein  composed 
entirely  of  calcite,  but  not  crumpled  and  deformed  as  are  the 
quartz  veins.  These  are  later  vein  deposits.  This  redeposited 
calcite  is  also  more  nearly  pure  calcite,  indicating  that  the  iron, 
to  a  considerable  extent,  rusted  out  as  solution  took  place.  In  these 
veins  may  be  found,  now  and  then,  quite  perfect  calcite  crystals. 
Mi  a  is  ^  ^n  contmumS  our  observations  beyond  the  bridge 
quartziteor  ^  we  find  that  the  rock  is  much  more  quartzose  than 
quartzose  mica  before — in  fact  much  of  it  may  be  called  a  micaceous 
quartz ite.  It  is  possible  that  this  part  of  the  cut- 
ting belongs  with  the  next  formation  to  be  described,  and  that 
here  is  where  the  one  begins  and  the  other  ends.  If  so,  this  is  a 
contact.  At  any  rate  the  two  are  so  closely  connected  that  we 
shall  make  no  serious  mistake  in  considering  this  rock  at  this  time, 
crum  lin  One  variety  is  of  a  paper  thin,  fissile  structure,  of 

in  micaceous      a  grey  color,  a  shade  or  two  lighter  than  the  normal 
quartzite.        schist,  and  somewhat  rusty  in  spots  due  to  the  rust- 
ing of  crystals  or  particles  of  iron  pyrites  distributed  through  the 
rock.    Another  variety  is  a  grey,  apparently  massive,  hard  quartzite, 
which,  on  freshly  broken  surfaces,  shows  very  little  trace  of  lamina- 


FOLDING  OF  PHYLLITE  AND  INCLUDED  QUARTZ  VEINS  AT  DEEP  CUT  NEAR 
SUMMIT  STATION. 


GEOLOGY   OF  WORCESTER.  25 

tion.  On  careful  inspection  this  variety  exhibits  a  crumpling  and 
crinkling  which,  for  fineness  and  delicateness,  are  simply  marvellous. 
This  is  specially  distinct  on  weathered  surfaces  because  the  edges 
of  some  of  the  laminae  are  more  rusty  and  are  thus  brought 
out  by  difference  in  color.  This  greater  rustiness  of  some  laminae 
is  due  to  the  impure  calcite  distributed  in  these,  as  is  shown  by 
the  effervescing  seen  under  the  magnifying  glass  when  the  rock 
is  touched  by  a  drop  of  hydrochloric  acid.  The  smooth  surface 
of  a  joint,  or  of  a  drill  hole,  may  present  a  wavy  structure  so  fine 
and  delicate  and  nicely  colored  as  to  rival  in  beauty  the  graining 
of  the  choicest  woods.  From  these  exceedingly  fine  folds,  we  ob- 
serve all  grades  up  to  the  coarse  ones  that  may  be  measured  by  feet. 
In  this  connection  we  may  see  how  difficult  it 

estimating       would  be  to  determine  the  thickness  of  this  formation. 

thickness  of      jt  is  qujte  evident  that  the  result  would  be  far  from 

this  formation. 

correct,  if  measurement  were  made  directly  across 
the  edges  of  the  laminae.  In  the  first  place  the  laminae  may  or 
may  not  be  parallel  to  the  original  stratification;  and,  in  the 
second  place,  because  of  the  crumpling  and  folding,  there  is  no  tell- 
ing how  many  times  the  same  laminae  are  repeated  within  a  given 
area.  For  this  reason,  wherever  we  give  the  estimation  of  the 
thickness  of  a  formation,  it  will  be  based  on  other  facts  than  the 
mere  measurement  across  the  edges  of  the  laminae.  At  the  best, 
an  estimation  of  the  thickness  of  the  phyllite,  which  will  be  given  in 
due  time,  will  be  but  approximate. 

Thus  far  by  the  study  of  certain  localities  we  have  attempted 
to  gain  ideas  generally  applicable  to  the  phyllite  as  a  whole.  There 
are,  however,  variations  in  it  which  are  only  local,  and  these  varia- 
tions are  well  worth  our  study,  if  we  wish  to  gain  a  complete  idea 
of  this  interesting  rock. 

Following  the  electric  car  road  from  College  street 
along  the  side-hill  on  the  way  to  Auburn,  we  are 
walking  over  this  same  phyllite,  as  we  may  easily 
see  from  numerous  outcrops.  After  we  have  gone  about  a  mile 
and  a  half,  there  appears  a  local  phase  of  this  rock  that  is  very 
noticeable,  and  at  times  beautiful.  Here  the  rock  has  a  light 
grey  color,  metallic  lustre,  smooth  greasy  feel,  but  does  not  leave 
a  mark  on  paper;  it  breaks  into  large,  flat,  thin  slabs,  the  surfaces 
of  which  are  finely  and  beautifully  ribbed.  This  delicate  ribbing, 
combined  with  the  metallic  lustre,  and  the  light,  almost  silvery, 


26  GEOLOGY   OF   WORCESTER. 

color,  gives  to  this  variety  a  special  beauty  not  often  seen  in  this 
rock.  The  ribbing  is  so  regular,  that,  at  first,  the  little  ridges  and 
furrows  seem  continuous,  lengthwise,  across  large  specimens.  On 
closer  examination,  the  ridges  and  furrows  are  seen  to  constantly 
fade  out,  and  others  as  imperceptibly  to  appear  parallel  to  the 
former,  thus  continuing  the  ribbing.  Nor  is  this  ribbing  all  of  the 
same  degree.  There  are  the  coarser  ridges,  but  a  small  fraction 
of  an  inch  in  height,  and  removed  from  the  adjacent  ridges  on 
either  side  by  but  a  small  part  of  an  inch;  on  these  are  imposed 
still  finer  undulations,  and  so  on  until  the  folds  are  too  fine  to  be 
seen.  It  is  in  fact  a  rock  surface  wrinkled  to  the  superlative 
degree.  But  what  does  this  wrinkling  mean?  It  tells  us  of  a 
secondary  change  within  this  rock.  After  the  ancient  sediments 
had  been  crystallized,  and  the  laminae  formed,  as  previously  ex- 
plained, this  rock  was  subjected  to  a  pressure  applied  along  the 
edges  of  the  laminae  and  in  the  plane  of  the  laminae,  at  right 
angles  to  the  direction  of  the  former  one  that  produced  the  laminae  ; 
and  the  rock,  possessing  little  strength  and  allowing  of  considerable 
freedom  of  motion  between  the  laminae,  yielded  in  infinitesimal 
folds  throughout  its  mass.  These  fine  folds  constitute  the  Beautiful 
ribbing.  But  these  minute  folds  have  a  deeper  meaning.  Look 
at  the  edge  of  a  specimen  where  you  see  the  ends  of  ridges  and 
Development  furrows.  There  are  lines  running  across  this  broken 
of  a  new  surface  where  successive  laminae,  one  above  the  other, 

have  either  been  pinched  out  to  nothing  or  broken  off. 
When  a  specimen  is  broken  across  the  laminae,  the  breaking  tends 
to  follow  these  lines.  These  are  lines  of  an  incipient  slaty  cleavage. 
These  little  folds  are  really  the  laminae  in  process  of  rotation  in 
minute  sections  into  a  plane  at  right  angles  to  the  original  foliation. 
This  rotation  stopped  at  just  the  right  stage  to  teach  us  a  most 
interesting  lesson.  Something  quite  similar  to  this  was  pointed 
out  near  the  east  end  of  the  railroad  cutting  at  Bloomingclale,  but 
in  the  case  of  the  latter  there  is  wanting  the  beauty  that  accom- 
panies the  former.  This  minute  wrinkling  of  the  schist  laminae 
is  quite  generally  characteristic  of  the  rock,  but  nowhere  else  is 
it  found  in  such  perfection  as  on  this  side-hill  in  Auburn. 

Continuing  on  in  our  journey,  following  the  electric 

car  track  to  the  road  leading  from  the  continuation 


ing  garnets       of  Southbridge  street  up  on  this  hill,  we  walk  along 
ana  stauroiite.     this  road  to  the  road  leading  ^  Auburn,  then  turn 


CRUMPLED  PHYLLITE  FROM  AUBURN.    ORIGINAL,  5  INCHES  BY  4. 


GEOLOGY    OF   WORCESTER.  27 

off  on  the  road  leading  to  John  C.  Maclnnes's  place,  and  find  ledges 
near  Mr.  Nye's  house.  The  rock  is  a  rusty  mica  schist  more  coarsely 
crystallized  than  is  the  normal  phyllite,  yet  a  part  of  the  phyllite 
band.  It  is  characterized  by  a  peculiar  odor,  when  breathed 
upon,  not  at  all  resembling  the  argillaceous  odor  common  to 
the  schist.  In  these  ledges  may  be  seen  many  little  garnets, 
one  every  now  and  then  showing  the  regular  form  of  a  rhombic 
dodecahedron.  This  figure  may  be  recognized  by  a  single  crys- 
talline face  having  the  shape  of  a  rhombus.  With  these  may 
also  be  seen  small,  dark  grey,  prismatic  crystals,  some  of  which 
cross  each  other,  or  branch  one  from  another.  These  are  crystals 
of  staurolite.  They  are,  now  and  then,  found  in  this  mica 
schist,  though  less  commonly  than  the  garnets.  The  latter  may 
also  be  found  in  considerable  abundance  in  the  outcrops  near  the 
centre  of  Auburn.  The  development  of  these  minerals  within  this 
comparatively  small  area  probably  indicates  that  beneath,  at  no 
great  depth,  is  a  mass  of  granite,  and  the  intrusion  of  this  in  a 
molten  condition  produced  a  little  higher  degree  of  metamorphism 
or  change  in  the  rock,  resulting  in  the  formation  of  these  minerals. 
The  study  of  these  phases  of  the  phyllite  will  certainly  repay 
us  for  an  afternoon's  journey  on  the  electric  car  over  this  Auburn 
hill.  Looking  to  the  north  from  this  hill  we  admire  the  beautiful 
view  of  the  hills  across  the  valley,  and  we  take  note  of  a  deep  notch 
in  the  horizon.  This  marks  an  ancient  river  valley,  the  valley 
now  occupied  in  part  by  the  Holden  reservoir. 

In  walking  over  the  side-hill  sloping  down  from 
^ne  Boston  and  Albany  railroad  to  Lake  View  one 
may  notice,  now  and  then,  bowlders  which  may 
possibly  attract  attention.  They  are  fragments  from  ledges  some- 
where to  the  north;  but  just  where  the  ledges  are,  is  not  known, 
though  probably  they  are  not  far.  The  rock  of  these  bowl- 
ders is  of  a  dull  dark  grey,  almost  black,  color.  It  is,  apparently, 
not  crystallized  so  much  as  is  the  normal  phyllite,  though  the 
many  little  glistening  points  show  that  the  crystallization  is  well 
advanced.  It  is  thin  bedded  in  structure,  the  laminae  averaging 
about  one  eighth  of  an  inch  in  thickness;  and  the  cleavage  between 
these  is  less  marked  and  distinct  than  in  the  phyllite  generally. 
Through  this  black,  dull,  laminated  mass  are  distributed  little, 
white,  prismatic  crystals,  about  as  large  in  diameter  as  a  fine  pin, 
and  from  one  eighth  to  one  half  inch  in  length.  These  crystals 


28  GEOLOGY   OF   WORCESTER. 

lie  in  planes  parallel  to  the  laminae,  and  are  generally  of  a  dull 
white  color.  Possibly  this  dullness  may  be  due  to  the  weathering 
which  the  bowlders  have  undergone.  These  little  white  crystals 
are  the  mineral  andalusite,  and  this  phase  of  the  phyllite  may  be 
called  an  andalusite  phyllite. 

This  phase  of  the  phyllite  reminds  us  of  the  beau- 
schist  of         tiful  chiastolite1  schist  which  occurs  so  abundantly 
2ncaste""a      in  bowlders  on  George  Hill  in  Lancaster.       The  mica 
part  of  tilis       schist  of  this  hill  is  a  northern  part  of  this  Worcester 
same  forma-      phyllite  area,  and  contains  large,  beautiful  crystals 
of  chiastolite  clearly  marked  by  distinct  crosses  ap- 
pearing in  the  ends  of  the  crystals.     These  crystals  are  frequently 
an  inch  in  diameter,  and  several  inches  in  length.     Moreover,  we 
may  notice  in  some  specimens  what  were  evidently  crystals  of 
chiastolite,  but  which  are  now  simply  masses  of  mica  preserving 
the  shape  of  the  chiastolite  crystals.     This  illustrates  how  one 
mineral  may  change  into  another,  while  imbedded  in  the  rock, 
assuming  the  crystalline  form  of  the  new  mineral  as  in  the  mica 
scales,  yet  the  whole  mass  of  scales  preserves  the  shape  of  the 
original  crystal. 

Anyone  at  all  interested  in  this  subject  should  not  be  without 
a  specimen  from  this  George  Hill  locality,  and  there  is  no  reason 
why  he  should,  for  nice  specimens  may  be  found  by  the  roadside 
and  in  the  stone  walls  on  almost  any  part  of  this  hill. 
Geoio  icaia  e  ^n  ^s  discussi°n  °f  the  Worcester  phyllite  there 
oftheworces-  is  one  point  which  we  have  thus  far  entirely  neg- 

ter   phyllite.        ^^       TQ    ^    geologist    the    study    of    ftny    rock> 

especially  if  it  be  a  sedimentary  rock,  or  a  recrystallized 
sedimentary  rock,  is  not  complete  until  he  knows  where  to 
place  it  in  the  earth's  history.  Through  many  changes  has  the 
earth  come  to  its  present  state.  There  has  been  a  development, 
both  animate  and  inanimate,  and  the  record  of  these  is  buried  in 
the  rocks.  From  their  study  must  this  history  be  revealed.  But 
this  history  is  long  and  complex;  so,  for  the  sake  of  simplicity 
and  system,  just  as  the  history  of  man  is  divided  into  various  ages, 
the  history  of  the  earth  is  divided  into  certain  periods.  The  names 
of  these,  as  adopted  by  the  geologists  of  the  U.  S.  Geological  Sur- 
vey, are,  beginning  with  the  earliest:  Archean,  Algonkian,  Cam- 

1  Chiastolite  is  a  variety  of  andalusite,  which  shows  a  cross  when  the  crystal  is  broken 
so  as  to  present  a  cross-section. 


GEOLOGY    OF   WORCESTER.  29 

brian,  Silurian,  Devonian,  Carboniferous,  Juratrias,  Cretaceous, 
Eocene,  Neocene,  Pleistocene.  These  periods  are  not  equal,  either 
in  length  of  time  represented  or  in  the  amount  of  the  earth's 
development  that  took  place  in  them.  The  older  or  earlier  periods, 
because  their  records  are  more  nearly  obliterated  and  less  easily  de- 
ciphered, represent  longer  durations  of  time  and  greater  changes; 
while  for  the  more  recent  periods  greater  detail  is  possible,  leading 
to  more  frequent  and  shorter  divisions  in  the  geological  history. 

Therefore,  before  we  leave  the  study  of  the  Worcester  phyllite, 
let  us  try  to  determine  where  it  belongs  in  this  scheme,  that  we 
may  know  what  relation  it  bears  to  the  development  of  the  earth. 
To  determine  this  in  the  case  of  a  rock  is  not  always  easy,  especial- 
ly in  the  case  of  a  recrystallized  sedimentary  rock,  for  the  records 
have  been  largely,  if  not  entirely,  destroyed,  during  recrystalli- 
zation.  In  spite  of  the  changes  through  which  this  phyllite  has 
passed,  fortunately  a  few  fossils  have  survived.  These  have  already 
been  described  in  our  discussion  of  the  rocks  found  at  the  coal  mine. 
These  fossils  fix  quite  definitely  the  period  in  the  earth's  history  to 
which  this  rock  belongs.  They  are  the  geological  coins.  They 
indicate  that  this  rock  belongs  to  the  Carboniferous,  and  thus  it  is 
brought  into  its  position  alongside  of,  or  near  to,  in  geological  time, 
the  rocks  containing  the  coal  beds  of  Rhode  Island  and  Pennsylvania. 
This  was  suspected  by  many  long  before  the  fossils  were  found. 
The  graphite  and  graphitic  anthracite  layers,  met  with  somewhat 
frequently  in  cutting  into  the  phyllite  here  in  Worcester  and  in 
other  places,  tell  of  a  large  amount  of  buried  vegetable  material, 
just  as  the  beds  of  coal  tell  of  an  ancient  vegetation.  Moreover 
the  degree  of  recrystallization  exhibited  by  the  Worcester  phyllite 
is  not  greater  than  that  exhibited  by  parts  of  the  Carboniferous 
of  Rhode  Island.  Considerations  like  these  led  Sir  Charles  Lyell, 
about  1843,  to  conclude  that  the  rocks  of  the  Worcester  coal  mine 
were  of  the  same  age  as  those  of  the  coal  formations  of  Rhode 
Island;  and,  undoubtedly,  the  same  idea  was  in  the  mind  of  Louis 
Agassiz  when,  standing  at  the  mouth  of  the  old  coal  mine,  he 
prophesied  that  some  day  fossils  would  be  found  there. 


CHAPTER  II. 
WORCESTER  QUARTZITE. 

The  phyllite,  thus  far  considered,  does  not  make  up  the  whole 
of  the  rock-floor  beneath  Worcester.  We  become  aware  of  this 
when  we  examine  the  ledges  outside  of  the  area  already  pointed 
out  and  indicated  by  the  blue  color  on  the  geological  map.  Let 
us  go  out  Highland  street  or  Pleasant  street  to  the  cemetery 
just  beyond  Newton  square  ;  there,  by  the  side  of  the  street  and 
in  the  open  space  opposite,  will  be  found  ledge.  A  glance  at 
this  is  sufficient  to  convince  us  that  it  is  not  the  phyllite.  We 
must  remember  that  this  ledge  is  an  index  of  what  is  beneath 
and  around  for  some  distance,  for  it  is  really  a  part  of  the  rock- 
floor.  Let  us  examine  this  rock  carefully  as  we  did  the  phyllite. 
Description  ^  *s  °^  a  dirty  grey  color  on  weathered  surfaces, 

of  micaceous      but  within  is  of  a  slightly  reddish  or  brownish  grey. 

quartzite.  jt    ^^    ^^    difficulty>    but    when    ft    doeg    break> 

the  pieces  fly,  showing  both  its  toughness  and  elasticity.  The 
surface  of  the  rock,  where  it  is  broken,  is  frequently  concave  like 
the  inside  of  a  shell,  on  account  of  which  we  say  that  this  rock  has 
a  conchoidal  fracture.  It  is  hard;  and  it  quickly  dulls  the  edge  of 
the  steel  drill.  It  would  furnish  excellent  macadam  for  our  streets. 
Under  the  magnifying  glass,  we  see  it  to  be  compact,  of  a  finely 
granular  texture,  and  made  up  of  glassy  quartz  and,  in  small  part, 
of  very  fine  scales  of  brownish  mica.  This  hard  rock  is,  then,  prac- 
tically made  up  of  finely  granular  quartz,  and  we  may  rightly  call 
it  a  quartzite.  We  notice  also  a  banded  structure,  the  layers 
varying  from  an  inch  or  so  to  many  inches  in  thickness  in  the 
weathered  ledge.  In  cutting  into  the  rock  the  workmen  find  it 
much  more  massive,  and  the  fissile  structure  less  clearly  marked. 
Frost  and  other  atmospheric  agents  bring  out  the  fissility  by  sepa- 
rating the  sheets  and  enlarging  the  divisions  between  them.  The 
layers  have  a  uniformity  in  position,  lying  parallel  to  each  other, 
though  we  may  here  and  there  find  unimportant  curves  and  bends. 
In  general  they  point  or  strike  nearly  north  and  south.  This 


LEDGE  AT  FOOT  OF  CHAD  WICK  STREET. 


GEOLOGY   OF   WORCESTER.  31 

fact  will  serve  as  an  index,  pointing  to  the  direction  in  which  we 
may  look  for  the  greater  extension  of  this  rock.  Moreover  the 
beds  are  almost  vertical  in  position,  having  a  dip  of  about  75  de- 
grees to  the  west.  Hold  a  closed  book  before  you,  resting  the 
back  on  a  horizontal  surface,  and  in  such  a  position  that  it  points 
5  degrees  east  of  north,  and  the  leaves  slope  down  to  the  west, 
making  an  angle  of  75  degrees  with  the  horizontal  plane;  the 
leaves  then  represent  quite  accurately  the  position  of  the  beds  of 
this  quartzite. 

But  we  know  somewhat  in  regard  to  the  extent 
Micaceous        of  this  quartzite  in  this  vicinity.     In  digging  the 
n'lath^Newton     trench  for  the  sewer  through  Pleasant  street,  west 
Hiii.  of  Newton  Hill,  and  also  in  the  western  part  of  High- 

land street  as  far  east  as  Park  avenue,  the  workmen 
dug  down  to  the  rock-floor  beneath,  and  found  this  floor  to  con- 
sist there  of  the  same  hard,  tough  quartzite.  As  the  beds 
beneath  one  street  are  parallel  with  those  under  the  other,  and 
point  towards  them,  we  may  safely  conclude  that  they  are  con- 
tinuous and  extend  under  Newton  Hill.  Having  found  that  this 
rock  makes  up  somewhat  of  the  rock-floor  beneath  Worcester,  let 
us  determine  accurately  its  extent.  It  has  been  pointed  out  that 
the  beds  point  in  a  northerly  direction.  Let  us  go  to  the  north 
in  search  of  outcrops  that  we  may  know  what  kind  of  rock  is 
there  in  the  rock-floor. 

Passing  out  Grove  street  to  the  corner  of  Chadwick, 
mimitrin  we  come  *o  an  excellent  outcrop.  The  rock  imme- 
Grove  street  diately  attracts  our  attention,  rising,  as  it  does,  in  a 
achadr"-ick°f  vertical  wall  12  feet  or  more  in  height  by  the  side  of 
the  street.  The  rock  is  made  up  of  clearly  marked 
laminae.  These  vary  in  thickness,  but  are  generally  not  more 
than  an  inch  thick.  In  some  cases,  where  the  rock  material  was 
favorable  for  such  development,  they  are  almost  paper  thin. 
Breaking  into  the  rock,  we  find  that  the  description  of  that  near 
the  old  cemetery  in  Pleasant  street  (see  page  30),  exactly  applies. 
It  has  the  same  color,  hardness,  toughness,  conchoidal  fracture, 
and  is  made  up  of  the  same  materials, — is,  in  fact,  the  same  rock. 
The  two  are  as  much  alike  as  they  might  be  if  they  were  not  more 
than  100  feet  from  each  other.  By  means  of  the  compass  we 
take  the  direction  of  the  strata,  and  find  that  they  point  25  to  30 
degrees  east  of  north.  They  dip  43  degrees  to  the  west.  We 


32  GEOLOGY    OF   WORCESTER. 

notice  a  decided  variation  in  the  position  of  the  beds  here  from 
that  of  the  like  beds  in  Pleasant  street.  There  can  be  no  reason- 
able doubt,  from  the  identity  of  the  two  rocks,  that,  if  the  loose 
covering  were  removed,  we  should  find  rock  like  these  two  out- 
crops making  up  the  rock-floor  between.  As  there  is  a  decided 
variation  in  the  strike  and  dip  at  the  two  places,  it  is  evident 
that  these  beds  do  not  always  follow  straight  lines,  with  uniform 
slant,  in  this  rock-floor,  but  curve  and  bend  in  wavy  bands. 

As  we  look  at  the  ledge  here  at  Chad  wick  street, 

Origin  of 

the  micaceous  we  are  impressed  with  the  nicety  or  perfection  of  the 
quartzite.  lamination.  On  breaking  into  it,  we  find  that  there 
are  two  other  facts  revealed.  There  are  cross-joints  running  in 
almost  every  direction  on  account  of  which  these  beds  break  into 
irregular,  sharply  angular  blocks  varying  greatly  in  size.  The  other 
fact  brought  to  light  is  that  the  material  in  some  beds  has  a  regular 
arrangement  in  bands  indicating  that  these  beds  are  made  up  of 
layers.  There  is  also  an  indication  of  a  layer  structure  on  a  larger 
scale  in  the  ledge  as  a  whole.  In  examining  the  rock  we  find 
these  variations  evidently  due  to  a  variation  in  the  relative 
abundance  of  quartz  and  mica.  One  stratum,  a  foot  or  more  in 
thickness,  is  thinly  fissile  and  very  micaceous;  the  adjacent  one 
is  thick  and  massive  and  lacking  in  mica.  In  this  way  the  ledge, 
as  a  whole,  may  be  divided  into  strata  which  may  be  traced  far 
into  the  ledge.  This  layer  structure,  indicated  by  the  banding, 
gives  us  the  first  chapter  in  the  history  of  this  rock.  This 
structure  clearly  points  back  to  a  time  when  this  rock  was  made 
up  of  strata  differing  slightly  in  composition.  They  must  have 
been  formed  by  currents  of  water  just  as  rock  strata  are  being 
formed  at  the  present  time.  When  the  current  can  no  longer 
carry  or  push  along  the  particles  of  broken  rock,  it  there  deposits 
them.  Every  little  brook  shows  us,  on  a  small  scale,  how  the 
larger  currents  do  this  work.  But  the  material  contained  in,  or 
constituting,  this  rock  tells  us  also  just  what  those  particles  must 
have  been.  The  great  mass  of  this  rock  is  glassy  quartz  with 
a  little  very  fine  mica.  The  quartz  tells  us  that  most  of  the  parti- 
cles deposited  by  those  currents  must  have  been  particles  of  quartz 
in  the  form  of  sand — grains  of  sand  rolled  and  washed,  just  as 
grains  of  sand  are  now  rolled  and  washed  along  the  shore.  The 
fine  mica  tells  of  a  small  amount  of  clayey  impurity  mixed  with 
the  sand.  But  the  rock  at  present  shows  us  nothing  appearing 


PART  OF  LEDGE  AT  FOOT  OF  CHADWICK  STKEET,  SHOWING  A  FAULT. 


GEOLOGY    OF   WORCESTER.  33 

like  grains  of  sand ;  and  the  mica  is  in  no  respect  like  particles 
of  clay.  Though  the  quartz  is  granular,  it  does  not  present  the 
granular  appearance  of  grains  of  sand.  It  is  too  compact  and  fine; 
it  has  rather  the  granular  texture  produced  by  crystallization,  and 
the  mica  scales  are  certainly  minute  crystals.  This  rock,  which  is 
clearly  crystalline  in  texture,  bears  or  preserves  the  indubitable 
marks  of  a  sedimentary,  fragmental  rock.  It,  then,  like  the 
phyllite,  must  have  been  partly  recrystallized;  but  instead  of  beds 
of  clay  or  mud  at  the  beginning,  it  consisted  of  beds  of  sand. 
These  passed  through  a  crystallization,  like  that  of  the  schist,  under 
the  influence  of  heat,  moisture  and  pressure.  The  grains  of  sand 
were,  in  part,  dissolved  in  the  hot  moisture  under  pressure,  yet 
not  sufficiently  dissolved  to  obliterate  the  original  layer  structure 
of  the  sand.  In  due  time  the  dissolved  quartz,  being  no  longer 
kept  in  solution  through  the  cooling  of  the  rock  and  the  relief 
from  pressure,  recrystallized,  filling  the  interstices  between  the  un- 
dissolved  grains,  and  the  clayey  impurities  crystallized  into  mica. 
When  a  very  thin  and  transparent  section  of  the  rock  is  exam- 
ined under  the  microscope  the  original,  angular  grains  may  still 
be  seen  in  part. 

..     Fault  Another  fact  may  be  noticed,  in  looking   at  this 

ledge,  which  shows  clearly  that  there  has  been  differ- 
ential motion  here.1  The  edges  of  the  beds  on  the  right  slope  up 
diagonally  across  the  face  of  the  ledge  in  somewhat  wavy  lines; 
while  on  the  left  the  beds  form  a  flattened  arch  with  the  edges 
almost  horizontal,  and  the  arch  pitches  down  to  the  south  at  an 
angle  of  about  20  degrees.  The  beds  of  this  arch  are  not  parallel 
with  those  of  the  ledge  generally,  and  the  region  where  the  two 
meet  near  the  middle  of  the  figure  gives  evidence  of  a  breaking  and 
crushing  of  the  rock.  It  is  evident  that  one  part  of  this  ledge  has 
been  moved  against  the  other  part.  When  there  is  a  break  in  a  rock 
in  this  way,  and  the  beds  on  one  side  do  not  agree  with,  or  match, 
those  on  the  other,  there  is  said  to  be  a  fault.  In  the  midst  of  the 
rock  on  the  Chadwick  street  side  of  this  ledge  may  be  seen  some- 
thing similar,  where  a  part  of  the  beds,  possibly  less  resisting  or 
weaker,  were  folded,  while  the  stronger  beds  on  either  side  were  not 
folded.  In  other  words  we  find  in  this  small  outcrop  abundant 
evidence  that  this  rock  mass  has  not  moved  as  a  unit,  but  one  part 
has  had  one  motion,  and  another  part  another  motion,  producing 
breaks  and  want  of  uniformity  in  position  of  adjacent  strata. 
1  See  plate  opposite. 
4 


34  GEOLOGY   OF   WORCESTER. 

Leaving  this  outcrop  at  Chadwick  street,  we  pass 
st*N  farther  north  to  the  ledges  appearing  in  Dodge 
Park.  This  rock  at  first  appears  quite  different  from 
that  last  examined.  It  shows  none  of  the  bedding  so  noticeable 
in  that,  but  breaks  into  exceedingly  irregular,  angular,  generally 
small,  pieces.  On  this  account  it  is  difficult  to  obtain  specimens 
of  good  shape.  On  breaking  into  this  rock,  we  find  it  of  a  grey 
color,  hard,  compact,  destitute  of  lamination,  but  cut  by  fissures 
in  every  direction  with  no  regularity.  There  may  be  seen  also 
innumerable,  fine,  at  times  capillary,  veins  of  quartz  running 
through  this  rock.  So  abundant  are  these  as  to  put  one  in  doubt 
as  to  which  constitutes  the  larger  part  of  the  ledge,  the  veins  or 
the  original  rock  substance.  Surfaces  of  the  ledge  frequently  pre- 
sent a  perfect  network  of  these  veins  where  the  weathering  has 
removed  the  softer  rock  material.  As  we  are  studying  these  under 
the  magnifying  glass,  our  attention  is  attracted  by  little  shining 
particles  of  iron  pyrites.  To  these  is  due  the  iron  rust  so  abundant 
in  the  fissures  and  noticed  when  the  rock  is  broken.  Taken  as  a 
whole  this  rock  is  a  grey  quartzite,  in  composition  resembling  that 
in  the  ledges  by  the  side  of  Chadwick  and  Pleasant  streets.  It  dif- 
fers from  those  most  markedly  in  lacking  the  laminated  structure. 
Possibly  it  had  that  structure  at  a  former  time;  but  if  so,  since 
then,  it  has  undergone  a  violent  disturbance  by  which  it  was  com- 
pletely shattered  into  fine  fragments,  and  afterwards  solidified  by 
the  deposition  of  quartz  in  the  fissures.  It  may  be  that  we  are  in 
error  in  considering  this  ledge  as  belonging  with  those  observed  in 
Pleasant  and  Chadwick  streets,  but  if  so,  the  error  is  not  serious, 
and  will  not  generally  affect  our  conclusions. 

In  this  way,  by  examining  the  ledges  one  after 
another  to  the  north,  we  may  find  what  part  of 
the  rock-floor  is  composed  of  quartzite,  and  we  may 
also  draw  quite  accurately  by  means  of  the  outcropping  ledges 
and  the  direction  of  the  beds  or  laminae,  a  line  separating  the  area 
of  phyllite  from  that  of  quartzite.  In  like  manner  we  may  work 
to  the  south.  Putting  these  results  together  we  may  clearly  see 
that  this  quartzite  forms  a  band  in  the  rock-floor  extending 
parallel  with  that  of  the  phyllite  and  lying  to  the  west  of  it.  So 
intimately  associated  are  these  two  that  whenever  one  is  found 
the  other  is  found  east  or  west  of  it.  That  part  of  Worcester 
under  which  the  band  of  quartzite  extends  is  represented  on 


GEOLOGY   OF   WORCESTER.  35 

the  geological  map  as  accurately  as  we  have  been  able  to  deter- 
mine it. 

Phyiiitebe-  From  this  digression,  to  point  out  the  extent  of  this 
and1  North*6  quartzite  band,  we  resume  our  study  of  the  ledges, 
Parks.  passing  from  Dodge  Park  to  North  Park.  On  the 
way  we  note  the  ledges  appearing  by  the  side  of  the  street  leading 
to  Burricoat  street.  In  these  ledges  we  at  first  notice  considerable 
reddish  grey  quartzite,  practically  the  same  as  that  we  have  been 
studying;  but  this  grows  less  abundant  as  we  proceed  until  the 
rock  is  all  phyllite.  We  have  crossed  the  line  between  the  two. 
But  here  the  line  is  not  clear  and  distinct,  the  reason  for  which 
will  appear  when  we  consider  the  relation  of  these  two  rocks  to 
each  other.  We  take  the  dip  and  strike  of  the  laminae,  and  find 
them  pointing  about  north  and  south  and  having  but  a  slight  slant 
or  dip  of  20  degrees  to  the  west.  We  contrast  this  flatness  with 
the  almost  vertical  position  of  the  phyllite  laminae  in  other  places 
where  we  have  studied.  All  these  facts  must  be  taken  into  con- 
sideration as  we  attempt  to  interpret  the  geology  of  this  region; 
and  our  work  is  not  complete  until  we  have  fitted  all  together  as 
the  parts  of  a  puzzle. 

crushed  Passing  into  Burncoat  street  and  turning  to  the 

granite  in  south,  we  notice  a  rock  in  the  gutter  of  the  street. 
North  Park.  It  .g  neither  of  the  rockg  thug  far  stu{jied.  Continu- 
ing, and  turning  into  North  Park,  in  the  vicinity  of  the  boathouse, 
we  find  more  of  this  peculiar  rock.  We  are  able  to  go  around 
the  area  in  which  this  rock  is  found,  as  it  extends  but  a  little  out- 
side of  the  park.  This  area  is  indicated  on  the  geological  map. 
This  rock  is  of  a  slightly  brownish  grey  color;  has  a  marked  foliated 
structure,  tends  to  break  in  thin  sheets  with  somewhat  irregular 
surfaces.  These  surfaces  are  of  a  brownish  color,  and  present  an 
abundance  of  thin  brownish  mica  as  a  veneering.  This  is  about 
all  the  mica  there  is  in  the  rock.  It  is  this  arrangement  of  the 
mica  that  gives  the  foliation  to  the  rock.  These  surfaces  of  the 
folia  also  present,  under  the  glass,  the  appearance  known  as  slicken- 
sides.  The  surfaces  are  polished  and  marked  by  fine  striations 
parallel  to  each  other,  as  if  there  had  been  a  sliding  of  one  sur- 
face on  the  other.  Looking  at  the  cross-section,  we  see  that  the 
rock  is  of  medium  fine,  granular  texture  and  contains  an  abun- 
dance of  quartz,  white  and  glassy.  There  is  also  much  feldspar, 
as  is  indicated  by  the  shining  cleavage  surfaces  which  flash  in  the 


36  GEOLOGY   OF   WORCESTER. 

sunlight.  Most  of  the  feldspar  is  so  fine  as  not  to  be  easily  dis- 
tinguished from  the  quartz,  but,  here  and  there,  is  a  well  defined 
particle,  the  largest  being  an  eighth  or  a  sixteenth  of  an  inch 
through.  Some  of  the  feldspars  are  lens-shaped.  Under  the  glass 
may  be  seen  little,  dark  grey  particles,  probably  magnetite,  as  there 
is  a  considerable  quantity  of  magnetic  material  in  the  powdered 
rock.  In  the  rock,  but  not  of  it,  are  also  many  coarse  veins  of 
glassy  quartz.  This  rock  is  at  first  very  perplexing,  as  it  resem- 
bles the  neighboring  rocks  only  in  its  foliated  structure.  Con- 
sidering the  composition  of  this  rock  it  is  essentially  a  mixture 
of  feldspar  and  quartz,  if  we  leave  out  of  consideration  the  veneering 
of  mica  on  the  folia.  Occurring  as  it  does  in  this  small  area, 
without  any  connection  with  the  neighboring  rocks,  its  composition 
and  occurrence  indicate  that  it  is  a  granite.  But  a  granite  of 
itself  would  not  have  this  foliated  structure,  nor  would  the  mica 
be  arranged  in  this  wray.  It  is  more  than  a  simple  granite.  The 
foliation  and  fragmental  granular  texture,  rather  than  crystalline 
granular  texture,  and  the  veneering  of  mica,  all  point  to  a  dynamic 
change  or  rearrangement  within  this  rock.  It  has  been  crushed, 
and  so  thoroughly  that  only  now  and  then  did  a  feldspar  escape. 
The  pressure  producing  this  crushing  was  not  parallel  to  the  folia, 
but  across  them  at  a  larger  or  smaller  angle,  thus  producing  a 
shearing  motion  within  the  rock-mass  by  which  the  substance 
was  arranged  in  folia.  At  the  same  time  these  folia  rubbed,  one 
against  another,  producing  the  slickensides,  and  transforming  the 
feldspar  on  the  surface  into  a  veneering  of  mica.  This  strange 
rock  is,  then,  a  crushed  and  sheared  granite. 
Micaceous  Having  digressed  from  our  study  of  the  quartzite 

quartzite  in  to  study  this  small  area  in  the  park,  we  continue  our 
observations,  and,  near  the  southeast  corner  of  the 
park,  come  to  a  rock  that  looks  familiar.  It  is  grey  in  color,  and 
laminated ;  the  edges  of  the  laminae  point  thirty-five  degrees  east  of 
north  and  dip  fifty-two  degrees  to  the  west.  Breaking  off  a  piece 
from  the  ledge,  the  rock  proves  to  be  the  slightly  brownish  grey  mica- 
ceous quartzite.  But  we  carefully  note  that  between  this  and  the 
band  of  quartzite  already  traced  to  the  west  is  a  band  of  phyllite 
appearing  in  the  outcrops  noticed  as  we  approached  Burncoat 
street.  This  is  also  indicated  on  the  geological  map.  Before 
trying  to  solve  this  problem  and  interpret  the  meaning,  let 
us  continue  our  observations,  because  we  remember  that  along 


GEOLOGY    OF   WORCESTER.  37 

Lincoln  street,  as  we  go  down  the  hill  towards  the 
Linartn  street  P°or  Farm,  we  have  seen  many  ledges.  Going  to 
these,  we  find  them  to  be  the  same  brownish  grey  mica- 
ceous quartzite,  but  a  little  less  hard  and  firm,  especially  near  the 
weathered  surfaces.  Another  fact  noticed  is  that  some  of  these  sur- 
faces are  irregularly  pitted.  These  cavities  sometimes  reach  an  inch 
into  the  rock,  and  may  be  an  inch  and  a  half  long  by  a  half  inch 
wide.  These  are  seen  only  on  weathered  surfaces,  and  we  immedi- 
ately attribute  them  to  weathering.  But  why  should  one  part 
of  this  rock  be  affected  so  much  more  than  another?  We  break 
into  the  rock  to  reach  that  which  the  agents  of  the 
a^r  nave  n°t  yet  acted  on.  The  rock  within  is  not 
uniform.  There  are  small  masses  of  a  soft,  white 
mineral  corresponding  in  size  and  shape  to  the  cavities  on  the 
weathered  surfaces.  A  drop  of  hydrochloric  acid  on  this  white 
mineral  tells  us,  by  the  effervescing,  that  the  mineral  is  calcite. 
The  explanation  of  the  cavities  is  now  clear.  Calcite  is  quite 
readily  soluble  in  water  containing  carbonic  acid,  and  hence  is 
dissolved  by  the  rain  water  falling  on  these  rock  surfaces,  leaving 
the  cavities  which  the  calcite  filled. 

There  are  here  also  many  veins  of  white  glassy 

Fine  quartz  J  J 

veins  in  the  quartz,  and  these  are  frequently  so  abundant  and 
quartzite.  gne  &s  ^Q  make  a  perfect  network  or  lacework  through 
the  rock.  These  veins,  as  has  been  pointed  out  before,  are  simply 
filled  cracks  or  fissures.  These  show  clearly  that,  at  some  stage 
in  the  history  of  this  rock,  it  was  subjected  to  some  disturbance 
which  completely  shattered  it. 

Here  also  we  note  the  direction  of  the  laminae,  for 

Variation  in  . 

strike  of  the     this  may  help  us  in  solving  problems,  before  we  are 
quartzite  in       through  with  the  subject.     Near  the  top  of  this  hill 

Lincoln  street. 

the  direction  is  thirty-five  degrees  east  of  north,  and 
the  dip  is  fifty-four  degrees  to  the  northwest;  part  way  down  the 
hill  the  strike  is  forty-three  to  forty-five  degrees  east  of  north,  and 
the  dip  forty-eight  degrees  to  the  northwest;  still  farther  down 
the  hill  the  strike  is  fifty  degrees  east  of  north,  and  the  dip  forty- 
seven  degrees  to  the  northwest.  As  we  go  down  this  hill  in  Lin- 
coln street,  approaching  the  Poor  Farm,  the  rock  laminae  point 
more  and  more  to  the  east.  We  are  evidently,  then,  following  a 
curve  in  the  rock  laminae,  which  we  should  find  continuous  were 
the  loose  material  removed  so  that  we  could  see  the  rock-floor. 


38  GEOLOGY   OF   WORCESTER. 

This  has  its  meaning  which  will  appear  in  due  time  as  we  proceed 
in  the  study. 

Moreover,  as  we  examine  carefully  and  study  the  rocks  in  this 
area,  in  one  part  of  the  ledge,  where  the  laminae  point  fifty  degrees 
to  the  east  of  north,  we  observe,  in  some  of  the  freshly  broken 
rock,  a  distinct  banding.  This  is  due  to  a  slight  variation  in  the 
relative  quantities  of  quartz  and  mica  constituting  the  rock.  This 
variation  and  banding  probably  point  back  to  the  layers  in  the 
sand  out  of  which  this  rock  was  formed.  These  bands  are  not  here 
parallel  with  the  sheets  into  which  the  rock  splits,  but  cut  across 
them,  having  a  direction  more  nearly  north.  Hitherto,  where  band- 
ing has  been  observed,  the  banding  and  splitting  have  been  paral- 
lel (the  layers  so  formed  are  called  laminae),  having  a  common 
direction  of  about  thirty  degrees  east  of  north.  This  exception 
brings  out  clearly  a  secondary  splitting  caused  by  pressure,  that 
may  cut  across  the  bedding,  and  the  sheets  so  formed,  which  are 
called  folia,  may  be  just  as  firm  and  strong  as  when  they  are  parallel 
with  the  bedding.  The  foliation  is  later  and  independent  of  the 
bedding,  and  is  due  to  pressure  and  a  rearrangement  of  rock 
particles  as  the  rock  underwent  changes  in  position. 

Without  attempting,  at  this  time,  to  explain  and 
interpret  all  of  the  facts  which  may  shed  light  on 
this  area,  we  turn  into  the  fields,  and  go  up  on  Mill- 
stone Hill,  passing  to  the  west  of  the  summit,  near  the  shore  of 
the  pond  on  Green  Farm.  Here  we  find  numerous  outcrops  in  the 
form  of  a  light  grey,  thinly  foliated  quartzite,  wrhich  is,  at  times, 
quite  micaceous.  It  is  not  unlike  the  quartzite  found  along  Lincoln 
street.  We  also  take  the  direction  of  the  laminae,  and  find  they 
strike  sixty  degrees  east  of  north,  and  dip  thirty-nine  degrees  to  the 
northwest.  If  we  compare  this  direction  with  that  in  a  like  quartz- 
ite found  just  northwest  of  the  coal  mine,  on  the  other  side  of 
the  hill,  where  the  laminae  point  fifty-five  degrees  west  of  north 
and  slant  forty-four  degrees  northeast,  and  then  think  of  what 
would  be  observed,  if  we  could  study  the  intervening  rock-floor  by 
ledges  as  we  did  the  rock-floor  down  Lincoln  street,  we  can  see  that 
we  should  find  the  laminae  gradually  changing  in  strike  from 
sixty  degrees  east  of  north  to  fifty-five  degrees  west  of  north,  and 
the  direction  of  the  dip  or  slant  from  northwest  through  north 
to  northeast.  In  other  words  the  laminae  curve  or  wrap  around 
the  northern  side  of  this  hill.  The  curve  in  the  laminae  in  the 


GEOLOGY    OF   WORCESTER.  39 

Lincoln  street  area  was  parallel  with  this,  only  a  little  more  removed 
from  the  hill. 

Quartzite  in  ^e  now  con^nue  our  observations,  going  towards 
East  Kendall  Belmont  street,  and  keeping  to  the  level  of  the  pre- 
vious observations  on  the  side  of  the  hill.  We  fre- 
quently find  the  same  quartzite  at  the  surface,  and  a  good  place 
for  us  to  stop  for  observations  again  is  at  the  foot  of  East  Kendall 
street.  Here  is  the  quartzite,  again  of  a  slightly  reddish  or  brown- 
ish grey  color,  and  thinly  laminated.  Again  we  take  the  strike  of 
the  laminae,  and  find  them  pointing  seventy  degrees  east  of  north, 
and  dipping  eighty  degrees  to  the  south.  Here  is  a  decided  change 
in  the  dip.  At  the  foot  of  the  last  or  most  southerly  pond  on 
Green  Farm  the  laminae  were  dipping  to  the  northwest,  while  here 
they  dip  to  the  south.  If  now  we  place  one  hand  with  its  back 
sloping  off  to  the  northwest  and  pointing  about  sixty  degrees  east 
of  north,  and  the  other  hand  near  the  first  and  pointing  seventy 
degrees  east  of  north  with  its  back  slanting  down  eighty  degrees 
from  the  horizontal,  it  will  be  seen  that  the  two  hands  are  parts 
of  a  fold  which  has  a  gentle  slope  to  the  northwest  and  a  steep 
slope  to  the  south.  In  other  words,  in  this  short  space,  we  have 
crossed  a  fold  in  the  beds  of  the  quartzite.  Doubtless  there  are 
numerous  other  folds  like  this  which  we  are  not  able  to  trace  out  so 
clearly,  for  this  quartzite  has  been  marvellously  folded,  as  we  shall 
see  before  we  are  through  with  its  study.  But  there  is  another  im- 
portant fact  for  us  to  notice  here  at  the  foot  of  East  Kendall  street. 
Following  the  direction  of  the  laminae  easterly,  and  only  a  few  feet, 
we  suddenly  come  to  an  entirely  different  rock.  This  we  recog- 
Keiation  of  nize  as  a  part  of  the  granite  of  Millstone  Hill,  of  which 
the  quartzite  we  snan  speak  in  due  time.  In  our  minds,  then,  we 
of  Millstone  can  see  what  would  be  exposed,  if  the  loose  earth  of  the 
Hiu-  street  were  removed — the  laminae  of  the  quartzite 

striking  directly  against  the  granite  of  Millstone  Hill,  and  the  granite 
attached  to  the  quartzite  securely,  yet  the  line  between  the  two  clear 
and  distinct.  Doubtless  we  should  be  able  to  trace  this  quartzite, 
bearing  the  same  relation  to  the  neighboring  granite,  south  across 
Belmont  street,  thence  around  the  foot  of  Normal  School  Hill,  if 
it  were  not  for  the  covering  of  loose  earth  spread  over  it.  This 
quartzite  probably  underlies  East  Worcester;  and  how  far  up 
towards  the  centre  of  the  city  it  extends  cannot  be  positively  stated, 
but  at  the  time  of  the  excavation  of  the  cellar  of  the  new  city 


40  GEOLOGY   OF  WORCESTER. 

hall,  the  rock-floor  there  exposed  was  this  same  quartzite.  Rely- 
ing on  this  we  represent  the  quartzite  on  the  map  as  extending 
under  the  city  as  far  as  that  point.  But  returning  to  East  Worces- 
ter and  following  around  Normal  School  Hill,  only  a  spur  of  Mill- 
Quartziteat  stone  Hill,  at  the  corner  of  Hunt  and  Shrewsbury 
the  comer  of  streets,  and  also  on  the  opposite  side  of  Shrewsbury 
Shrewsbury  street,  we  find  ledges  having  a  familiar  appearance. 

streets.  The  rock  is  massive,  without  lamination,  but  cut  by 
joints  crossing  each  other  in  various  directions.  Along  these  joints 
the  rock  has  weathered  away  leaving  creases  in  its  surface.  On 
the  outside  this  rock  is  of  a  dark,  dirty  grey  color;  but  within 
is  of  a  light  greenish  grey,  sparkling  with  little  black  crystals  of 
ottrelite.  Under  the  magnifying  glass,  the  rock  is  a  mass  of  fine, 
glassy  particles  of  quartz. 

Breccia  ^  ^ew  hundred  yards  east  and  southeast,  on  the 

near  shrews-  border  of  this  quartzite  area,  and  evidently  closely 
eet'  connected  with  the  quartzite,  is  another  rock  worthy 
of  considerable  study.  It  may  be  found  by  the  side  of  the  dummy 
railroad  track,  and  also  between  this  track  and  the  tracks  of  the 
Boston  and  Albany  railroad.  These  ledges  are  not  a  part  of  the 
railroad  cutting  already  studied  under  the  Carboniferous  phyllite, 
but  are  nearer  the  city.  The  rock  is  of  a  dirty,  dark  grey  color, 
has  a  noticeable  lamination,  so  that  we  may  take  the  dip  and 
strike.  We  find  this  rock  pointing  thirty-five  degrees  east  of  north, 
and  the  laminae  standing  almost  vertically,  but  dipping  seventy- 
eight  degrees  to  the  southeast.  If  we  compare  the  position  of 
these  laminae  with  that  of  the  phyllite  laminae  just  east  in  the  rail- 
road cutting,  we  find  that  while  the  two  agree  in  direction,  those 
of  the  former  dip  to  the  southeast  and  those  of  the  latter  to  the 
northwest.  If  you  will  place  your  hands  so  that  they  point  thirty- 
five  degrees  east  of  north,  and  the  right  hand  slopes  down  or  dips 
seventy  degrees  to  the  northwest,  while  the  left  dips  seventy-eight 
degrees  to  the  southeast,  the  relation  of  the  laminae  in  these  ad- 
jacent rocks  will  clearly  appear.  The  hands  slope  towards  each 
other,  and  indicate  the  trough  of  a  fold.  There  is  then  in  the 
rock-floor  here  a  fold  with  the  concave  side  of  the  fold  up;  this 
is  a  synclinal  fold.  It  is  another  evidence  of  the  folding  that  these 
rocks  have  undergone. 

But  to  return,  in  thought,  to  the  rock  itself,  let  us  carefully 
examine  it.  It  has  a  blotched  appearance,  the  blotches  being 


I 


PART  OF  SURFACE  OF  LEDGE  OF  BRECCIA.  WITH  GLACIAL  MARKS  EXTEND- 
ING DIAGONALLY  ACROSS. 


GEOLOGY   OF   WORCESTER.  41 

several  shades  lighter  than  the  general  surface  of  the  rock.  There 
is  no  regularity  in  the  shape  of  these.  Some  are  long  and  narrow 
with  rounded  ends;  some  are  irregularly  diamond  shaped  with 
sharp  angles;  some  are  as  wide  as  long,  and  partly  angular  and 
partly  rounded.  On  the  surface  of  this  rock  may  be  seen  in  places 
the  glacial  scratches  pointing  seven  degrees  east  of  south.  On 
breaking  into  this  rock  so  as  to  examine  a  fresh  surface,  we  find 
the  rock  within  much  like  that  outside,  only  a  little  darker.  The 
blotches  consist  of  fine,  granular  quartzite,  of  a  grey  color,  closely 
resembling  the  quartzite  already  described.  In  fact  the  quartzite 
of  these  blotches  and  that  of  the  ledges  a  few  hundred  feet  away 
are  near  enough  alike  to  have  been  side  by  side  in  the  same  ledge. 
These  angular  fragments  of  quartzite  are  imbedded  in  a  darker, 
finely  grained  crystalline  matrix  consisting,  as  far  as  can  be  seen, 
of  little,  black,  lengthened  crystals  of  biotite  or  ottrelite.  Such  a 
rock,  made  up  of  angular  fragments  cemented  by  other  material,  is 
called  a  breccia;  but  as  the  cementing  material  has  been  crystal- 
lized, it  may  not  be  improperly  called  a  metamorphic  breccia. 

Relation  of  ^u*  **  *s  more  than  this  because  of  what  it  tells 

breccia  to  us  of  the  relation  of  the  rocks  we  are  considering, 
the  quartzite.  It  ie^s  of  the  breakmg  mto  pieces  of  the  neighbor- 
ing quartzite,  along  the  border,  after  the  layers  of  sand  from  which 
it  was  formed  had  been  changed  into  sandstone  or  quartzite ;  then 
of  the  forcing  of  these  fragments  into  the  adjacent  beds  of  clay, 
probably  already  hardened  into  shale;  and,  finally,  of  the  recrys- 
tallization  of  the  fragments  into  quartzite  if  they  were  not  already 
quartzite,  and  of  the  clay  or  shale  about  the  fragments  into  a 
crystalline  matrix. 

Quartzite  in  Continuing  our  study  around  the  border  of  Mill- 
the  "  coal  stone  Hill,  we  go  near  the  State  Lunatic  Hospital. 

Mme  area.  rpj^  onjy  jeciges  appearing  are  the  granite  of  Millstone 
Hill,  which  we  do  not  care  to  study  at  the  present  time.  The 
ledges  lower  down  on  the  hill  are  covered  until  we  come  to  Planta- 
tion street.  Here  for  a  short  distance  phyllite,  like  that  seen 
in  the  railroad  cutting  near  by,  appears.  North  of  the  asylum, 
though  the  rock  bordering  the  granite  is  not  seen  in  outcropping 
ledges,  the  numerous  fragments  of  quartzite  in  the  loose  material 
leave  little  doubt  but  that  this  is  the  rock  beneath  in  the  rock-floor. 
Thus  we  trace  the  quartzite  along  the  eastern  foot  of  Millstone  Hill 
to  the  area  northwest  of  the  coal  mine,  where  we  find  ledges,  the 


42  GEOLOGY   OF   WORCESTER. 

rock  of  which  is  identical  with  that  at  the  corner  of  Hunt  and 
Shrewsbury  streets.  It  is  of  a  light  greenish  grey  color,  composed 
almost  entirely  of  finely  granular,  glassy  quartz,  but  cut  in  every 
direction  by  almost  innumerable  veins  of  white,  glassy  quartz. 
These  veins  vary  in  thickness  from  an  inch,  or  a  little  more,  down 
to  those  of  capillary  fineness.  As  we  have  pointed  out  in  other 
cases,  these  veins  tell  us  of  a  tremendous  breaking,  even  shattering, 
of  this  rock,  and  then  of  the  deposition  of  quartz  within  the  fis- 
sures cementing  the  fragments  together  into  firm  and  massive 
rock.  The  quartzite  here  also  has  a  distinct  lamination.  The 
strike  is  fifty-five  degrees  west  of  north,  and  the  dip  forty-four 
degrees  to  the  northeast.  In  some  of  this  rock  there  is  a  clearly 
marked  layer  structure  consisting  of  alternating  light  and  dark 
green  bands,  some  not  more  than  one  eighth  of  an  inch  in  thick- 
ness. These  bands  point  back  to  the  original  layers  in  the  sand 
out  of  which  the  quartzite  was  formed,  and  indicate  the  original 
structure  of  the  rock.  Here  the  original  structure  and  the  second- 
ary structure,  represented  by  the  lamination,  are  parallel.  Here,  in 
spite  of  the  great  changes  the  rock  has  been  through,  the  two  agree 
in  direction,  while  in  Lincoln  street  we  found  the  one  cutting  across 
the  other.  It  all  depends  on  the  direction  of  the  applied  pressure. 
When,  as  is  generally  the  case,  the  original  sedimentary  lamination 
and  the  secondary  foliation  (dependent  on  the  position  of  the  mica 
scales),  coincide  we  may  assume  that  the  mica  crystallized  between 
the  original  laminae  under  the  pressure  of  the  superincumbent  beds 
before  the  folding,  while  the  folding  caused  the  crushing  and  quartz 
veining. 

Thence,  passing  across  the  fields,  we  come  to  Lincoln  street,  and 
have  encompassed  Millstone  Hill.  We  have  found  its  base  sur- 
rounded by  quartzite  reaching  somewhat  up  on  its  sides;  moreover 
we  have  found  the  quartzite  laminae  leaning  towards  the  hill  on  all 
sides  except  the  west  and  southwest,  where  they  strike  directly 
against  the  granite. 

But  this  band  of  quartzite  surrounding  Millstone 

Extension  of  .  •      i          i  • 

this  Mnistone     Hill  is  not  an  isolated  area;    it  is  the  southern  ex- 
urn  area  to       tremity  of  a  large  area  extending  several  miles  to 
the  north.     We  may  trace  this  area  by  going  into 
the  fields  north  of  Lincoln  street,  and  observing  the  ledges,  as  we 
go  along  to  the  north,  reaching  the  Boston  and  Maine  railroad  at 
the  large  curve  northeast  of  the  Summit;    thence  following  the 


GEOLOGY    OF    WORCESTER.  43 

railroad,  and  examining  the  ledges  in  the  hills  on  either  side,  we 
shall  find  the  rock  a  quartzite,  closely  resembling  that  which  we 
have  been  studying,  but  frequently  pure  white  in  color.  It  is  made 
up  of  glassy,  granular  quartz.  This  quartzite  we  may  follow  until 
it  disappears  beneath  the  sands  and  gravels  of  the  Nashua  River 
Beyond  these  it  again  appears;  and  is  found  in  the  upper  part 
of  the  high  hill  just  east  of  Oakdale.  It  will  be  well  for  us  to  make 
a  careful  study  of  the  rocks  of  this  hill,  for  here  are  facts  of  exceed- 
ing interest  revealing  the  relation  of  the  quartzite  to  the  Carbon- 
iferous phyllite,  which  we  have  already  studied.  Starting  from 
the  Oakdale  station  we  go  east,  and  ascend  this  hill.  Im- 
mediately we  find  outcropping  ledges  of  phyllite  identical  with  that 
which  we  have  studied  farther  to  the  south  in  Worcester.  In  fact 
by  ledges  appearing  here  and  there  we  may  trace  the  connection 
of  this  phyllite  east  of  Oakdale  with  that  in  Worcester,  and  thus 
show  a  continuous  band.  This  band  of  phyllite  is  parallel  to,  and 
west  of,  the  quartzite  band  already  traced  from  Worcester  to  Oak- 
dale.  These  are  shown  on  the  geological  map. 

To  continue  our  observations  on  this  side  hill,  we 
antfcihle  take  ^ne  strike  and  dip  of  the  laminae,  and  find  them 
pointing  fifteen  to  twenty  degrees  east  of  north,  and 
dipping  thirty-five  degrees  or  more  to  the  west.  We  then  ascend 
the  hill  still  farther,  and,  near  the  top,  find  more  outcrops.  The 
rock  in  these  is  quartzite,  not  phyllite.  We  also  take  the  strike 
and  dip  of  the  laminae  in  these  ledges.  They  point  twenty-five 
degrees  east  of  north,  and  dip  about  fifteen  degrees  to  the  west. 
In  other  words  the  quartzite  is  sloping  down  under  the  phyllite 
found  in  the  lower  part  of  the  hill.  Continuing  still  farther  to 
the  east  over  this  hill,  we  find  the  quartzite  in  the  ledges,  but, 
after  passing  the  first  road  extending  north  and  south,  we  notice 
a  change  in  the  position  of  the  laminae.  While  they  still  point 
in  about  the  same  direction,  they  slant  or  dip  to  the  east,  or  in 
the  opposite  direction  from  what  they  did  in  the  western  part  of 
the  hill.  On  going  still  farther  to  the  east,  we  find  the  phyllite  also 


44  GEOLOGY   OF   WORCESTER. 

dipping  to  the  east  at  an  angle  of  about  forty-five  degrees.  We 
may  represent  in  the  accompanying  sketch  what  we  have  found 
in  going  over  the  hill. 

The  phyllite  leans  against  the  quartzite  on  the  west  and  on  the 
east;  and  the  quartzite  in  the  western  part  of  the  hill  leans  towards 
the  east,  and  in  the  eastern  part  of  the  hill  towards  the  west.  Put- 
ting together  these  facts,  it  is  evident  that  we  may  represent  the 
laminae  as  constituting  an  arch  or  fold.  But  as  this  rock-floor 
has  been  cut  into  and  worn  away  by  the  rivers  of  past  ages,  this 
fold  is  not  now  perfect;  the  phyllite,  that  once  extended  over 
from  the  eastern  side  to  the  western  above  the  quartzite,  has  been 
worn  away  leaving  only  these  remnants  on  the  eastern  and  western 
slopes.  These  are  quite  sufficient,  however,  to  plainly  show  us  the 
structure.  When  rocks  are  bent  into  a  fold  or  arch  like  this,  they 
constitute  an  anticline  or  anticlinal  fold. 

With  this  key  to  the  problem,  we  now  retrace  our 

Millstone        steps  from  this    hill  across    the   Nashua  river,  and 

Hill  — Oakdale 

anticline.  examine  the  quartzite  ledges  observed  in  our  former 
study,  as  we  went  to  the  north.  We  now  find  the 
anticlinal  fold  traceable  through  the  whole  area;  and,  here  and 
there,  at  the  very  crest  of  the  fold,  we  also  find  small  patches  of 
phyllite  which  have  fortunately  escaped  the  erosion,  and  which  tell 
us  plainly  that  the  phyllite  once  covered  the  whole  of  the  quartzite, 
thus  completing  the  fold.  The  structure  of  this  band  of  quartzite 
thus  becomes  clear  until  we  reach  that  part  encircling  Millstone 
Hill.  But  what  we  have  seen  to  the  north  helps  us  to  understand 
also  this.  When  this  fold  was  formed,  so  great  was  the  strain  upon 
the  quartzite  and  phyllite  in  the  Millstone  Hill  area,  that  they  were 
broken  in  the  bending,  and  on  the  east  of  this  break  were  uplifted 
in  a  fold  so  as  to  rest  on  the  granite ;  while  west  of  the  break  these 
rocks  were  not  uplifted,  or  only  slightly  so,  and  the  ends  of  their 
laminae,  left  by  the  break,  directly  adjoined  the  granite.  At  the 
same  time,  in  the  eastern  and  northeastern  parts  of  this  hill, 
the  laminae  were  shattered,  producing  the  broken  structure  already 
pointed  out.  What  connection  the  granite  may  have  had  with 
all  this,  we  shall  attempt  to  point  out  in  another  chapter. 

In  encompassing  Millstone  Hill,  we  did  not  stop  at 
Wigwam  Hill.     The  rock  in  this  hill  is  worthy  of 
more  than  passing  mention.     As  we  go  to  the  southern 
crest  of  the  hill,  we  note  the  position  of  the  laminae  or  folia  which 


PART  OF  THE  SUBFACE  OF  THE  LEDGE  AT  WIGWAM  HILL.    THE  FOLDED 
BANDS  OF  QUARTZ  SHOW  THE  ORIGINAL  STRUCTURE;  THE  STRUC- 
TURE EXTENDING  OBLIQUELY  FROM  TOP  TO  BOTTOM  OF  THE 
PLATE  is  THE  SECONDARY  STRUCTURE. 


'JKOI.OGY    OF    WORCESTER.  4.5 

point  or  strike  about  twenty-eight  degrees  east  of  north,  and  are 
practically  vertical.  The  rock  is  of  a  dirty,  more  or  less  rusty, 
grey  color  with  an  exceedingly  ragged,  uneven  surface.  On  exami- 
nation we  find  this  raggedness  due  to  the  fact  that  there  are  two 
kinds  of  rock  substance  here, — one  of  quartz  in  irregular,  wavelike 
bands,  which  project  beyond  the  general  surface;  and  the  other, 
that  in  which  these  bands  are  imbedded.  The  latter  is  a  mica 
schist.  It  is  softer  and  is  more  easily  removed  by  the  atmospheric 
agents,  leaving  the  edges  of  the  irregularly  wavy  bands  projecting 
and  forming  the  uneven  surface.  These  quartz  bands  are  gen- 
erally of  a  faint  reddish  tint  on  the  weathered  surface,  and  this 
coloring  makes  them  more  noticeable.  The  most  noticeable  fact 
here  is  the  complexity  of  the  folding  seen  in  the  quartz  bands. 
If  you  will  think  of  wet  paper,  pliable  and  yielding,  spread  in  a 
large  sheet  on  the  top  of  a  table,  and  then  of  two  of  its  opposite 
edges  pushed  towards  each  other,  the  many  folds,  which  this  wet 
paper  will  then  assume,  will  hardly  outnumber  those  that  may  be 
counted  in  these  bands  of  quartz  distributed  through  this  schist. 
Fourteen  folds  were  counted  in  a  distance  of  sixteen  inches  in  one 
band.  We  may  trace  large  folds  whose  sides  may  measure  a  foot 
from  crest  to  hollow;  then  we  may  see  that  these  sides  are  folded 
into  smaller  folds  measuring  but  an  inch  or  so;  and  in  some  cases 
we  may  distinguish  folds  in  the  sides  of  these  smaller  ones  down 
to  the  almost  infinitesimal  folds. 

Moreover  the  folding  is  not  all  that  may  be  here  observed.  In 
tracing  these  quartz  bands  we  observe  that  they  are  frequently 
pinched  out  until  they  end  in  knife  edges;  and  then,  with  a  few 
inches  of  schist  intervening,  again  appear  and  continue  in  a  like 
crinkled  condition  until  again  pinched  out  or  cut  off.  There  are 
thus  produced  faults  almost  without  number.  In  places  also  may 
be  observed  the  opposite  of  pinching  out — a  longitudinal  compres- 
sion by  which  the  band  has  been  thickened,  especially  at  the  crest 
of  folds.  In  fact  so  complex  has  been  the  folding,  crumpling, 
crinkling,  faulting  and  cutting  of  these  quartz  bands  that  it  is 
with  difficulty  that  the  observer  can  trace  with  certainty  the  same 
band  many  feet. 

These  quartz  bands  are  composed  of  finely  granular  quartz,  and 
are  easily  distinguished  from  glassy  quartz  veins  of  which  there 
are  some  distributed  through  this  schist.  The  rock  in  which  the 
quartz  bands  are  is  a  sandy  mica  schist,  thinly  laminated,  and 


46  GEOLOGY   OF   WORCESTER. 

shows,  at  first  sight,  very  little  of  this  folding.  This,  however,  is  not 
as  great  a  contradiction  as  it  at  first  appears.  On  the  weathered  sur- 
faces, we  find  that  the  weathering  serves  to  bring  out  the  structure 
in  places,  and  reveals  a  folding  within  the  schist  quite  as  wonder- 
ful as  that  within  the  quartz  bands.  At  times  the  laminae  of 
the  schist  fold  in  and  out,  thus  conforming  with  the  quartz  bands ; 
again  by  pressure  a  new  structure — a  slaty  cleavage — is  produced 
at  an  angle  with  the  lamination  and  the  folia  project  directly  in 
between  the  arms  of  a  fold  of  a  quartzite  band,  the  ends  of  the 
laminae  striking  plumb  against  the  concave  surface  of  the  fold, 
again  the  laminae  of  this  schist  cut 
through  the  quartz  band  by  faulting 
so  that  there  may  be  several  inches  of 
schist  between  the  ends  of  the  quartz 
band. 

We  have  thus  in  detail  described  the  facts  observable  at  Wigwam 
Hill,  that  we  may  the  better  picture  in  our  minds  the  conditions 
that  must  have  prevailed  when  this  folding  took  place.  We  find 
here  no  evidence  of  breaking,  not  even  in  the  brittle  quartz. 
There  are  neither  cracks  nor  fissures  indicating  the  relieving  of 
tension  on  the  outside  of  these  folds;  there  is  no  puckering  on 
the  inside;  neither  does  the  soft  schist  seem  to  have  had  any 
difficulty  in  cutting  through  the  harder  quartz.  Everything  points 
to  a  soft  mass  made  yielding  and  plastic  by  enormous  pressure 
under  which  the  strength  and  hardness  and  brittleness  of  the 
quartz  bands  counted  for  naught,  and  the  whole  was  as  pliable 
as  so  much  dough  or  putty.  Nevertheless  so  gentle  were  all  of 
the  movements  that  the  bands  of  quartzite  and  schist  did  not 
mingle.  While  in  this  condition,  by  chemical  action  between  the 
particles,  the  mica  scales  of  the  mica  schist  were  formed,  and,  as 
they  were  formed,  grew  in  the  direction  of  least  pressure,  as  has 
been  before  explained,  thus  producing  the  schistose  structure. 
After  the  mica  scales  had  been  formed,  the  folia  of  the  schist  were 
in  part  folded  very  complexly  as  may  be  observed  on  weathered 
surfaces. 

We  may  go  back  in  the  history  of  this  rock  and  think  of  it  as 
made  up  of  layers  of  sand — now  and  then  a  layer  of  pure  sand, 
the  other  layers  of  sand  mixed  with  much  mud  or  clay.  When 
this  material  was  recrystallized.  the  pure  quartz  layers  underwent 
the  least  change,  and  are  hence  most  easily  traced_at  the  present 


GEOLOGY    OF    WORCESTER.  47 

time.  These  quartzite  bands,  then, .  preserve  for  us  the  original 
bedding  of  this  rock  which  would  otherwise  have  been  entirely 
obliterated  during  the  crystallization  and  folding.  In  taking  the 
direction  and  slant  of  the  folia  at  this  ledge,  we  must  of  necessity 
look  at  this  secondary  structure  because  the  original  structure  is 
so  confused  as  to  make  the  taking  of  the  dip  and  strike  of  the 
original  structure  impossible.  We  could  not  have  a  better  illus- 
tration of  the  way  in  which  secondary  structure  may  supersede 
original  structure  in  a  metamorphosed  sedimentary  rock. 

While  we  are  studying  the  rocks  at  Wigwam  Hill 
warping  in  there  is  another  fact  worthy  of  careful  thought.  Let 
structure  of  ug  compare  the  position  of  the  folia  in  this  hill  with 

quartzite  in  L 

"  coal  Mine"      that  of  the  laminae  in  the  coal  mine  area.     Here  at 
area.  Wigwam  Hill  the  rock  folia  are  pointing  twenty- 

eight  degrees  east  of  north  and  are  practically  vertical,  while  in 
the  quartzite  a  few  hundred  feet  northwest  of  the  coal  mine  the 
strike  of  the  beds  is  fifty-five  degrees  west  of  north  and  the  dip 
forty-four  degrees  northeast.  It  is  evident  that  the  rock  folia  of 
one  do  not  agree  in  position  with  the  laminae  of  the  other  in  these 
neighboring  localities;  but  the  lamination  of  the  one,  parallel  to 
the  original  bedding,  and  the  foliation  of  the  other,  cutting  across 
the  original  bedding,  are,  in  these  recrystallized  metamorphic  rocks, 
due,  in  large  part,  to  one  and  the  same  cause,  pressure  with  or 
without  shearing.  When  the  pressure  was  at  right  angles  to  the 
bedding,  the  lamination  is  parallel  to  the  bedding;  when  the  pres- 
sure was  not  at  right  angles  to  the  bedding,  the  foliation  is  not 
parallel  to  the  bedding,  but  cuts  across  the  bedding  at  some  angle. 
It  follows  from  this  that  if  the  rock  surface  between  these  two 
neighboring  localities  were  exposed  to  view,  we  should  find  either 
the  foliation  that  appears  in  Wigwam  Hill  diverging  less  and  less 
from  parallelism  with  the  original  structure  as  we  go  from  Wigwam 
Hill  to  the  quartzite  northwest  of  the  coal  mine;  or  else  we  should 
find,  in  the  uncovered  rock  surface,  a  line  of  separation  between  the 
two  structures — on  the  west  of  this  line  the  lamination  parallel  with 
the  original  bedding,  and  on  the  east  the  foliation  cutting  across 
the  bedding.  Which  of  these  really  occurs  is  not  possible  to 
determine,  because  the  rock  surface  is  so  well  covered.  If  it  is 
the  latter,  that  is,  a  sharp  line  or  narrow  zone  between  the  two 
structures,  then  there  is  undoubtedly  a  fault  there;  the  change 
from  one  to  the  other  is  marked  by  a  breaking  of  the  rock.  But 


48  GEOLOGY    OF   WORCESTER. 

as  the  rock  was  in  an  exceedingly  yielding  or  plastic  condition,  as 
is  evidenced  by  the  quartz  bands  in  Wigwam  Hill,  it  is  more 
likely  that  the  former,  that  is,  the  gradual  transition  from 
one  structure  to  the  other,  would  be  found  in  the  uncovered 
rock  surface;  and  we  may  call  this  a  warping  of  lamination  into 
foliation. 

We  have  already  pointed  out  a  sharp  synclinal  fold  between  the 
rocks  just  west  of  the  railroad  cutting  near  Bloomingdale  and  the 
rocks  of  the  cutting  itself;  this  syncline  continued  to  the  north 
extends  into  this  region  between  Wigwam  Hill  and  the  coal  mine 
area,  and  is  the  southward  extension  of  the  disagreement  that 
there  is  between  the  structure  of  the  rock  northwest  of  the  coal 
mine  and  the  structure  of  the  rock  in  Wigwam  Hill. 

As  we  study  this  rock  at  Wigwam  Hill,  the  question  may  arise 
whether  we  shall  classify  this  rock  with  the  quartzite  or  with  the 
phyllite.  Tracing  the  ledges  to  the  north  and  to  the  south  from  this 
hill,  we  notice  that  in  some  respects  the  rock  comes  to  resemble  the 
latter;  but,  taken  as  a  whole,  considering  its  quartzitic  composition, 
it  seems  best  to  classify  it  with  the  quartzite.  Moreover  followed 
to  the  south,  it  joins,  as  we  have  pointed  out,  the  quartzose  band 
just  west  of  the  bridge  at  Bloomingdale;  and  in  the  fields  east 
of  Plantation  street  occurs  more  of  this  same  micaceous, 
sandy  quartzite,  highly  crumpled,  lying  east  of  the  well  defined 
phyllite;  and  again,  farther  to  the  south,  a  like  micaceous,  highly 
folded  quartzite  is  found,  even  to  the  Quinsigamond  area.  On 
going  from  Wigwam  Hill  to  the  north,  we  may,  in  like  manner, 
trace  this  band  of  quartzite,  always  lying  east  of  the  Carboniferous 
phyllite,  as  far  as  we  have  traced  the  latter  even  to  the  northern 
boundary  of  the  state;  but  we  find,  as  we  go  to  the  north,  that  the 
micaceous  quartzite  becomes  a  more  nearly  pure  quartzite  indis- 
tinguishable from  the  quartzite  of  the  other  bands  already 
described.  This  is  also  represented  on  the  geological  map. 

Let  us,  then,  accurately  arrange  in  our  minds  the 

Relation  of 

the  quartzite      salient  facts  that  have  been  brought  out  in  regard  to 
ami  phyllite       ^he  areas  of  quartzite  in  the  Worcester  region,  es- 
pecially in  the  northern  part  of  Worcester.      Com- 
mencing in  the  western  part  of  Worcester,  as  far  as  we  have  now 
studied,  and  going  east,  there  is  a  band  of  quartzite,  then  a  band 
of  phyllite,  then  a  second  band  of  quartzite,  then  a  second  band 
of  phyllite,  then  a  third  band  of  quartzite.     These  bands  are  ap- 


GEOLOGY   OF   WORCESTER.  49 

proximately  parallel,  and  the  phyllite  in  the  two  bands  is  identical, 
and  the  quartzite  in  the  three  bands  of  quartzite  is  practically 
identical.  It  is  reasonable,  then,  to  suspect  some  relation  between 
the  two  phyllite  bands  and  among  the  three  quartzite  bands.  In 
the  high  hill  cast  of  Oakdale  we  have  seen  the  quartzite  dipping 
under  the  phyllite  on  the  west,  and  also  on  the  east ;  it  must  be  then 
that  the  quartzite  which  dips  down  under  the  phyllite  comes  up  to 
the  surface  again  east  and  west  of  the  phyllite,  under  which  it  goes, 
thus  becoming  the  eastern  and  western  bands  of  the  quartzite. 
Adding  then  to  our  illustration  on  page  43  and  extending  it  to 


A— B+c— D=phyllite.  K — A.  B— c,  D— F=quartzlte. 

the  east  and  the  west,  we  may  represent  the  quartzite  extend- 
ing beneath  the  phyllite,  and  thus  the  bands  of  quartzite  are 
simply  parts  of  one  underlying  mass  of  quartzite.  South  of 
Worcester,  and  north  from  this  Oakdale  region,  the  central 
band  does  not  appear,  because  the  phyllite  is  continuous  from 
the  western  quartzite  band  to  the  eastern.  The  phyllite  has  not 
been  removed  by  erosion  down  to  the  quartzite.  At  any  point, 
then,  where  the  phyllite  appears,  if  a  boring  were  made  through 
it,  the  drill  would  strike  the  quartzite.  This  is  the  relation  in 
position  that  these  two  rocks  bear  to  each  other. 

Seeing,  as  we  have,  that  the  quartzite  everywhere 
te*  extends  beneath  the  Carboniferous  phyllite,  it  is 
quite  natural  for  us  to  desire  to  know  how  thick 
this  phyllite  is.  It  is  certain  that  this  covering  of  phyllite  is  not 
of  uniform  thickness  any  more  than  are  the  glacial  deposits  above 
the  ledges.  There  are  small  areas  of  this  phyllite  on  the  crest  of 
the  anticlinal  fold,  which  have  been  pointed  out,  and  the  extent 
of  these  areas  may  be  easily  expressed  in  square  feet.  The  thick- 
5 


50  GEOLOGY   OF   WORCESTER. 

ness  of  the  phyllite  in  such  areas  must  be  expressed  by  a  few  inches 
or,  at  the  most,  by  a  few  feet.  The  surface  of  the  quartzite  be- 
neath the  whole  of  the  phyllite  is  far  from  a  uniformly  level  surface 
from  the  very  nature  of  the  changes  that  the  rock  has  been  through. 
In  the  folding  and  crumpling  the  surface  must  have  been  bent 
into  elevations  and  depressions,  and  the  phyllite  folded  down  into 
the  quartzite,  and  the  quartzite  folded  up  into  the  phyllite.  This 
is  probably  the  reason  why  we  at  times  find  the  quartzite  in  the 
midst  of  the  phyllite,  as  we  do  just  west  of  the  bridge  at  Blooming- 
dale,  in  the  railroad  cutting.  If  then  the  surface  of  the  earth  had 
been  cut  down  to  a  level  plain,  still  the  phyllite  would  be  of  vary- 
ing thickness  because  of  this  interfolding  of  the  two.  Because  of 
the  inequality  in  the  erosion  the  problem  becomes  still  more  com- 
plicated to  estimate  the  thickness  of  the  overlying  phyllite.  In 
but  few  localities  are  we  able  to  give  any  definite  idea  of. its  exact 
thickness.  Where  we  crossed  the  phyllite  in  going  from  Dodge 
Park  to  North  Park,  it  is  doubtless  thin,  not  more  than  fifty  feet 
at  the  most,  more  likely  twenty-five  feet.  In  general  we  may  say 
that  the  thickness  of  the  phyllite  may  be  expressed  in  hundreds, 
rather  than  thousands,  of  feet;  in  other  words,  it  is  comparatively 
thin. 

Age  of  Before  leaving  the  quartzite  we  must  attempt  to 

the  micaceous  put  it  in  its  proper  place  in  the  historical  series.  In 
Zlte'  studying  this  quartzite  we  have  found  it  intimately 
associated  with  the  Carboniferous  phyllite,  interfolded  with  it 
so  complexly,  frequently,  that  the  two  cannot  be  represented 
separately  on  our  map,  because  of  the  smallness  of  the  scale.  The 
quartzite  extends  alongside  of  the  phyllite  throughout  the  extent 
of  the  latter,  and  does  not  extend  to  any  distance  beyond  it  at  the 
end.  The  degree  of  crystallization  appearing  in  the  quartzite  is 
just  about  the  same  as  that  found  in  the  phyllite.  There  is  no 
marked  unconformity  between  the  two  so  far  as  can  be  made  out, 
though  in  a  region  of  such  complex  folding  and  crumpling,  where 
frequently  the  only  structure  to  be  made  out  is  that  due  to  pressure 
and  crystallization,  it  is  quite  possible  that  there  may  be  a  minor  un- 
conformity which  has  not  yet  been  determined.  Taking  into  con- 
sideration these  facts  and  comparing  the  quartzite  with  the  neigh- 
boring crystallines,  it  seems  best  to  consider  the  quartzite  as 
belonging  to  the  same  age  as  that  to  which  the  phyllite  belongs, 
though,  as  it  occurs  beneath  the  phyllite,  it  is,  of  course,  a  little 


GEOLOGY   OF   WORCESTER.  51 

earlier  or  older.  Because  of  the  close  relationship  of  these  two 
rocks  in  position,  and  because  of  the  common  changes  of  folding 
and  recrystallization  through  which  they  have  passed,  we  conclude 
that  the  quartzite,  like  the  phyllite,  belongs  to  the  Carbonifer- 


A  hill  of 


CHAPTER  III. 
GRANITE  OF  MILLSTONE  HILL. 

In  our  study  thus  far,  we  have  gone  around  Mill- 
granite  stone  Hill.  Let  us  now  ascend  this  hill.  It  makes 
no  difference  on  which  side  we  begin  the  ascent,  we 
shall  find  the  encompassing  quartzite  giving  place  to  the  granite 
in  the  upper  part  of  the  hill.  On  the  south  side  the  granite  is 
met  at  the  very  base;  on  the  east  side  the  granite  appears  in  the 
grounds  of  the  State  Lunatic  Asylum,  and  extends  nearly  to  the 
foot  of  the  hill  between  there  and  the  coal  mine;  on  the  north 
side  the  ledges  are  well  covered  so  that  little  of  the  granite  is  seen, 
and  that  not  as  far  north  as  the  coal  mine;  on  the  west  side,  the 
quartzite  extends  well  up  on  the  hill  in  the  vicinity  of  the  pond 
on  Green  Farm,  and,  farther  south,  in  East  Kendall  street  and 
the  neighboring  streets;  thence  south  the  ledges  are  well  covered 
until  those,  of  Normal  School  Hill,  which  are  of  granite,  appear. 
These  last  are  simply  a  part  of  the  great  granite  mass  of  Millstone 
Hill,  and  we  may  think  of  the  quartzite  as  being  at  the  base  of  this 
hill  in  East  Central  street.  So,  ascending  the  hill  in  any  direction, 
we  shortly  meet  the  granite  which  constitutes  the  great  mass  of 
this  hill. 

This  rock  may  be  best  studied  in  the  quarries  at  the  top  of  the 
hill.  The  extensive  quarrying  has  exposed  broad  areas  of  the 
granite,  and  the  unweathered  rock  is  ready  for  our  study.  It  is 
well  for  us  to  trim  out  a  rectangular  piece,  four  inches  by  three, 
as  a  typical  specimen. 

This  granite  is  very  different  from  the  phyllite  or 
the  quartzite.  It  frequently  presents,  on  weathered 
surfaces,  a  very  rusty  appearance;  has  a  massive 
structure ;  is  wanting  in  foliation  or  lamination ;  is  cut  by  fissures  or 
cracks,  called  joints,  in  various  directions  into  irregular,  angular 
blocks.  Looking  now  at  the  fresh  surface  of  the  specimen  we  have 
trimmed,  we  see  the  rock  is  of  a  light  grey  color,  and  of  a  medium 


GEOLOGY   OF   WORCESTER.  53 

coarse,  granular  texture.     The  grains  vary  greatly  in  appearance,  so 
let  us  study  each  kind  by  itself,  constantly  using  the  hand  magnify- 
ing glass.     As  we  move  the  specimen  in  the  light,  numerous  bright, 
Feldspar         shining,  smooth  surfaces  flash.     These  are  cleavage 
surfaces,   and   indicate  the   crystalline  structure  of 


dime.  ^is  mineral,  and  also  of  the  rock.  This  mineral  is 
white  in  color,  porcelain-like  in  lustre,  and  evidently  makes  up  a 
considerable  part  of  this  rock.  It  is  feldspar,  and  generally  ortho- 
clase  or  potash  feldspar.  If,  however,  we  carefully  examine  many 
of  these  cleavage  surfaces  under  the  glass,  we  find,  now  and  then, 
one  crossed  by  many  fine,  straight,  parallel,  hair-like  lines.  These 
are  really  fine,  smooth  grooves  or  ridges  in  the  cleavage  surface, 
and  are  formed  by  very  thin  plates,  with  slanting  edges,  placed 
in  succession,  so  that  the  slanting  edges  either  slope  towards  each 
other,  producing  a  furrow  or  groove,  or  away  from  each  other, 
producing  a  very  fine  ridge.  This  arrangement  is  due  to  the 
crystallization,  and  is  characteristic  of  the  lime  and  soda  bearing 
feldspars.  The  orthoclase  feldspar  never  presents  these  striations 
on  its  cleavage  surfaces,  whereas  the  plagioclase  or  triclinic  feld- 
spars frequently  do.  We  therefore  conclude  that  this  granite  of 
Millstone  Hill  contains  principally  orthoclase  feldspar,  together 
with  a  much  smaller  amount  of  plagioclase  feldspar. 

The  next  mineral  is  clearly  distinguished  from  the 
"blue'  ami  *'     porcelain-like    feldspar  by  its  glassy  lustre,  by  its 


smo°th,  irregular  surfaces  resembling  the  surfaces  of 
broken  glass  particles,  and  by  its  smoky  color.  On 
searching  carefully,  examining  one  after  another  of  these  glassy 
particles,  we  may  find  some  having  a  decided  blue,  and  others 
having  a  faint  amethystine  tint.  If,  perchance,  we  are  examining 
a  somewhat  weathered  surface,  we  may  find  almost  every  one  of 
the  glassy  particles  blue  instead  of  smoky  ;  within  the  rock,  however, 
where  the  weathering  has  not  gone,  more  of  these  particles  are 
smoky.  It  is  thus  evident  that  the  weathering,  generally,  if  not 
always,  produces  the  blue  color  in  this  glassy  mineral.  It  may  be 
that  the  substance  producing  the  smoky  shade  is  oxidized  by  the 
oxygen  of  the  air  to  a  substance  of  a  blue  color.  What  this  sub- 
stance is,  has  not  been  determined,  but  from  the  somewhat  abun- 
dant occurrence  of  the  metal  manganese  in  this  rock,  we  have  been 
led  to  suspect  that  this  metal  has  something  to  do  with  the  develop- 
ment of  this  blue  color,  frequently  amethystine,  in  these  weathered 


54  GEOLOGY   OF  WORCESTER. 

surfaces.  The  glassy  mineral  is  quite  generally  distributed  in  some- 
what rounded  particles,  each  particle  a  distinct  individual,  not  a 
mass  of  granular  grains,  as  might  result  from  crushing.  This 
mineral  we  immediately  recognize  as  quartz,  which  is  frequently 
seen  in  mineral  veins  in  six-sided  prisms,  terminated  by  six-sided 
Quartz  crys-  pyramids.  In  fact  if  we  examine  many  of  these 
tais  in  the  quartz  particles  in  the  midst  of  the  granite,  we  shall 
granite.  ^  constantly  reminded  of  these  six-sided  prisms  or 
pyramids  by  the  cross-sections  presented  where  quartz  particles 
were  broken  off.  By  long  searching  we  may  find  a  little,  sharp, 
six-sided  pyramid  projecting  from  the  rock  surface.  It  is  a  crystal 
of  quartz  in  the  very  midst  of  the  granite.  But  much  more  fre- 
quent than  these  regular  crystals,  we  find  isolated,  more  or  less 
rounded,  particles  of  quartz  included  in  feldspar.  These  have 
been  called  anhedra,  because  lacking  the  definite  shape  of  perfect 
crystals.  Breaking  away  the  feldspar  around  them,  we  see  that 
they  have  a  dull,  etched  surface  like  the  surface  of  ground  glass. 
Occurring  in  this  way,  this  quartz  shows  that  it  did  not  crystallize 
later  than  did  the  feldspar,  but  either  before  or  along  with  the  lat- 
ter; while  the  irregular  coarse  quartz  filling  the  spaces  between  the 
feldspar  particles  solidified  after  the  feldspar  crystallized,  and  hence 
filled  the  spaces  then  remaining.  We  may  then  divide  the  quartz 
into,  as  it  were,  two  generations.  But  in  this  we  are  anticipating 
a  subject  about  which  we  shall  have  more  to  say  in  connection 
with  the  occurrence  of  the  granite  here.  The  second  abundant 
constituent  of  this  rock  is,  then,  quartz  or  silica. 

On  examining  the  rock  carefully,  especially  under 
the°granite       ^ne  S^SLSSJ  we  notice,  here  and  there,  but  nowhere 
abundant,  small  black  particles  made  up  of  little 
black  scales,  shining  brightly  with  a  submetallic  lustre.     This  min- 
eral is  biotite  or  black  mica. 

But  while  searching  for  these  black  particles,  in  which  we  may 
see  the  scales  distinctly,  we  must  be  on  the  watch  for  other 
particles,  if  perchance  there  be  any.  We  may  observe  small 
particles  of  a  slight  greenish  or  brownish  color,  made  up  of  many 
thin  sheets,  standing  on  edges  and  pressed  tightly  together.  We 
may  pick  these  sheets  apart,  and  then  we  see  nearly  colorless 
scales  resembling  the  biotite  scales,  except  in  color.  This  is  an- 
other mica,  muscovite  by  name. 


GEOLOGY   OF   WORCESTER.  55 

Purple  tinor  Before  this,  in  our  careful  searching,  our  attention 

spar  in  the  has  been  attracted  by  little  violet,  or  purple,  colored 
granite.  particles.  These  are  not  distributed  regularly  or 
abundantly;  and  we  may  not  find  even  a  single  one  on  the  surface 
of  some  good  sized  specimens,  but,  sooner  or  later,  we  shall  see 
such  a  particle,  as  the  light  strikes  it,  bringing  out  its  color.  If 
the  particle  is  large  enough  to  test,  we  find  it  quite  soft,  so  that  it 
is  easily  scratched  by  the  knife.  This  is  fluor  spar,  and  this  min- 
eral will  be  mentioned  in  another  connection,  before  we  are  through 
with  this  hill.  Its  occurrence  here  will  help  us  to  understand  its 
occurrence  in  the  other  manner  noted  beyond. 

Of  tho  five  minerals  found  in  this  rock  evidently  the  first  two 
are  the  essential  ones  on  account  of  their  abundance,  making  up, 
as  they  do,  probably  ninety-nine  hundredths  of  this  rock;  but  the 
micas  we  shall  also  consider  as  important,  though  not  abundant, 
while  the  fluor  spar  we  may  think  of  as  accidental.  All  these 
minerals  are  crystalline  in  structure,  and  as  they  constitute  the 
grains  of  the  rock,  we  may  describe  the  rock  as  of  a  wholly  crys- 
talline, granular  texture. 

Moreover  in  our  observations,  while  we  note  many  joints  and 
fissures  running  in  various  directions,  we  see  no  banding  in  the 
arrangement  of  these  minerals,  nor  any  foliation,  nor  any  tendency 
of  the  rock  to  break  along  certain  planes,  because  of  the  parallel 
arrangement  of  certain  minerals.  The  rock  is  perfectly  massive. 
All  of  these  characteristics  indicate  that  we  are  studying  a  granite. 
But  if  it  is  really  a  granite,  then  it  is  an  eruptive  rock.  In  this 
it  differs  from  the  rocks  hitherto  studied,  as  they  were  sedimentary, 
and  afterwards  recrystallized  into  their  present  condition.  When 
we  say  that  the  granite  is  eruptive,  we  mean  that,  in  the  form 
of  a  molten  rock-mass,  which  we  may  call  a  magma,  it  rose  from 
some  greater  depth,  into  its  present  position,  and  there  solidified, 
crystallizing  as  it  did  so,  into  this  massive,  crystalline  rock.  But 
Proofs  that  if  this  rock  thus  came  into  its  present  position,  there 
the  rock  of  are  certain  facts  that  we  may  reasonably  look  for  as 

Millstone  J  .... 

Hiii  is  confirming  this  idea.  Molten  rock  rising  in  this  way 
granite.  from  beneath  must  cause  more  or  less  disturbance 
in  the  neighboring  rocks,  either  melting  its  way  into  them,  or 
moving  them,  or  breaking  them  and  inclosing  fragments  of  them, 
or  forcing  its  way  into  fissures  within  them,  or  changing  them 
because  of  its  contact  with  them. 


56  GEOLOGY    OF   WORCESTER. 

We  will,  then,  in  our  study  look  for  these  facts, 
granitt?  'dikes  We  will  first  look  for  fissures  in  the  neighboring  rock 
wanting.  jn^o  wnich  this  molten  rock  may  have  flowed,  and 
there  solidified.  These,  if  there  be  any,  will  radiate  from  the  cen- 
tral mass;  and  hence  are  most  likely  to  be  found  near  where  the 
granite  and  neighboring  rock  come  together.  Unfortunately  for 
us  in  this  study,  the  bordering  line  is  generally  covered  by  glacial 
material,  and  there  is  but  a  limited  area  where  we  are  able  to  see 
these  rocks  near  to  each  other.  This  is  best  seen  in  the  vicinity 
of  East  Kendall  street.  Even  here  the  actual  contact  is  concealed. 
Nevertheless  we  are  able  to  determine  that  the  bordering  line  be- 
tween the  two  is  not  straight,  nor  a  regular  curve,  but  is  rather 
a  zigzag  line.  Though  this  is  true,  we  do  not  find  any  place  in 
the  quartzite  where  the  granite  has  flowed  into  a  fissure  and  there 
solidified.  However,  as  the  surface  material  is  removed  from  this 
region  in  the  laying  out  of  new  streets  and  in  the  digging  of  cellars, 
it  is  quite  probable  that  such  may  be  found.  We  are,  then,  forced 
to  look  for  other  facts  to  confirm  the  idea  that  this  rock  was  in 
a  molten  condition  when  it  came  into  its  present  position. 
Disturbance  We  therefore  seek  for  evidence  showing  that  the 

of  neighbor-  neighboring  rocks  have  been  disturbed.  It  is  not 
mg  rocks.  necessary  to  seek  far.  It  has  been  pointed  out  that 
the  laminae  of  the  quartzite  in  East  Kendall  street  strike  or  point 
directly  against  the  granite  of  Millstone  Hill.  Over  on  the  other 
side  of  the  hill  the  quartzite  again  appears,  but  leaning  against 
the  hill;  that  is,  the  laminae  of  the  quartzite  rest  on  the  granite 
instead  of  striking  against  it.  These  facts  we  have  illustrated  in 
Figure  A.  But  the  quartzite  on  one  side  of  the  hill  is  a  part  of 


EastftenJalt 
Quartzite 


Granite, 
ure    A. 


fig 

the  same  formation  of  which  the  quartzite  on  the  opposite  side 
is  a  part,  and  the  laminae  of  one  must  be  joined  with  those  of 


GEOLOGY   OF   WORCESTER. 


57 


the  other.  But  before  we  try  to  do  this  we  must  bear  in  mind 
that  this  area  has  been  subjected  to  great  erosion,  so  that  a  vast 
thickness  of  rock  has  been  removed  from  above  those  which  are 
now  at  the  earth's  surface.  The  quartzite  now  appearing  at  the 
surface  was  formerly  covered  by  a  considerable  thickness  of  phyllite. 
Moreover  this  Millstone  Hill  area  is  not  isolated,  but  is  part  of 
quite  an  extensive  anticline  which  we  have  already  traced  from 
Oakdale,  south,  to  this  region.  To  connect  the  laminae  of  the 
quartzite  on  opposite  sides  of  Millstone  Hill,  we  must  then  build 
up  the  anticline  out  of  the  material  that  has  been  removed  by 
the  erosion,  restoring  the  quartzite  and  phyllite  above  the  granite. 
But  in  doing  this  it  is  necessary,  not  only  to  fold  the  laminae,  but 
also  to  warp  them  so  that  those  almost  vertical  in  East  Kendall 
street  may  join  those  dipping  to  the  northeast  near  the  coal  mine. 
The  relative  positions  of  the  three  rocks,  granite,  quartzite  and 
phyllite,  before  erosion  removed  any,  and  the  possible  shape  of  the 
fold,  we  have  tried  to  illustrate  in  Figure  B. 


But  this  folding  and  warping  of  the  rocks  are  not  the  only  evi- 
dence of  disturbance  in  this  region.  We  have  already  pointed  out 
how  the  laminae  of  the  quartzite  wrap  around  the  northern  end 
of  Millstone  Hill,  indicated  by  the  change  of  strike;  we  have  also 
pointed  out  that  this  curving  in  the  strike  may  be  noticed  as  far 
away  as  in  the  Lincoln  street  area,  where  there  is  a  change  in  the 
strike  from  thirty-five  degrees  east  of  north  to  fifty  degrees  east 
of  north  observed  in  passing  down  the  hill.  The  meaning  of  this 
is  that  there  was  sufficient  pressure  exerted  against  this  rock  either 
to  warp  the  beds  of  the  quartzite  around  the  mass  of  crystalline 
rock  of  the  hill,  or  else  to  develop  in  the  quartzite  a  new  structure 
curving  around  the  same,  and  thus  indicating  a  pressure  normal 


58  GEOLOGY   OF   WORCESTER. 

at  all  points  to  this  curve.  \Such  a  pressure  would  more  likely  he 
from  within  outward,  than  from  outside  towards  the  centre. 

But  the  warping  and  rearrangement  of  the  particles  producing 
a  curvature  in  the  structure,  did  not  take  place  without  a  severe 
breaking  of  the  rock,  and  this  breaking  is  an  added  evidence  of 
disturbance.  It  was  pointed  out  that  the  quartzite  in  the  Lincoln 
street  area  showed  a  network  of  very  fine  quartz  veins;  that  in 
the  coal  mine  area  the  quartzite  presents  the  same,  and  that  the 
rock  of  the  coal  mine  has  been  broken,  and  then  the  fragments 
in  part  cemented  by  glassy  quartz.  Ml  these  facts  indicate  a 
shattering  of  the  rocks  in  the  area  bordering  this  hill;  and  the 
cementing  of  these  fragments  by  glassy  quartz  points  to  the  work 
of  hot  waters  carrying  silica  into  the  fine  cracks  and  crevices,  fill- 
ing them  and  making  the  rock  whole  and  strong  again.  Putting 
all  these  facts  together — the  hot  waters,  the  shattering,  the  new 
structure  or  warping — they  indicate  a  disturbance  exactly  accord- 
ing with  the  intrusion  of  a  heated  rock  mass  from  beneath  up 
into  the  quartzite  and  schist. 

inclusions  ^u*  we  are  no^  o^^g6^  to  stop  here  in  our  proof 

of  phyiute  in  of  the  way  in  which  this  crystalline  rock  of  Millstone 
Hill  came  into  its  present  position,  though  perhaps 
we  might  be  reasonably  sure  with  the  evidence  already  given. 
If  the  quartzite  and  phyllite,  at  greater  or  less  distances  from 
the  hill,  were  broken  and  shattered,  then  the  rock  nearer  and 
resting  on  the  crystalline  rock  must  have,  in  all  probability,  been 
also  broken  into  larger  or  smaller  fragments;  if  the  crystalline 
rock,  in  a  molten  condition,  came  into  contact  with  these  fragments, 
it  is  quite  probable  that  some  fragments  of  the  phyllite  and  quartz- 
ite became  included  in  the  molten  rock,  and  may  now  be  found 
in  the  midst  of  the  crystalline  mass. 

With  this  idea  in  mind  let  us  search,  if,  perchance,  we  may 
find  such.  The  best  place  in  the  hill  for  us  to  search  is  the 
quarries,  for  there  broad  areas  of  the  rock  are  laid  bare.  In  the 
western  part  of  the  large  quarry,  where  the  quarry  men  have  not 
recently  worked,  we  find  a  thin  band  of  rock,  evidently  not  the 
same  as  the  rock  of  the  quarry  and,  for  this  reason,  not  disturbed 
by  the  quarrymen.  This  band  is  but  a  few  inches  thick,  but 
many  feet  in  length;  it  extends  about  twenty-four  degrees  west 
of  north,  and  dips  to  the  east,  about  sixty-seven  degrees  from  a 
horizontal  position.  The  rock  is  soft,  of  a  dark  slate  color,  and 


GEOLOGY   OF   WORCESTER.  59 

thinly  laminated.  The  laminae  are  not  always  parallel,  because 
small  blocks  within  the  band  have  been  moved  out  of  the  general 
parallel  position,  and  in  these  the  laminae  are  at  various  angles 
with  the  laminae  in  the  remainder  of  the  band.  In  other  words, 
there  has  been  a  breaking  of  the  rock  within  this  band  into  frag- 
ments, in  places,  and  these  fragments  have  moved  out  of  their 
former  positions,  producing  a  breccia.  The  surfaces  of  the  laminae 
show,  in  general,  a  fine  folding  or  crinkling,  resembling  that  so 
characteristic  of  the  Worcester  phyllite.  In  fact,  so  close  is  the 
resemblance  of  the  rock  of  this  band  to  the  neighboring  phyllite, 
which  we  have  studied  in  the  first  chapter,  that  fragments  from 
it,  placed  beside  some  from  the  railroad  cutting  near  Bloomingdale, 
might  be  thought  to  have  come  from  the  same  ledge.  Moreover 
the  contact  between  this  phyllite  band  and  the  crystalline  rock  on 
either  side  is  a  contact  of  fusion — not  of  faulting  nor  of  sliding  of 
one  over  the  other — just  such  a  joining  of  the  two  as  would  result 
from  a  molten  rock  solidifying  in  contact  with,  and  attaching 
itself  to,  another  rock  not  molten,  as  the  molten  rock  cooled.  From 
this  study  we  are  obliged  to  conclude  that  phyllite  fragment 
or  band  must  have  been  included  in  the  crystalline  rock  when 
the  latter  was  in  a  molten  condition.  While  the  phyllite  was  not 
so  affected  by  this  contact  with  heated,  molten  rock  but  what  it 
is  easily  recognized,  the  crystalline  rock,  for  quite  a  distance  on 
either  side  of  the  phyllite,  is  dark  in  color,  as  if  the  molten  rock 
had  absorbed  coloring  material  from  the  included  phyllite. 

Moreover,  if  we  go  to  the  quarry  back  of  the  State 

Inclusion  J 

of  micaceous      Lunatic  Asylum,  whence  rock  was  obtained  for  those 
quartzite  m       buildings,  we  may  find  another  inclusion,  in  the  cen- 

granite. 

tral  part  of  this  quarry,  consisting  of  a  band  of  rock 
distinct  from,  and  unlike  the  crystalline  rock.  This  band  is  from 
six  inches  to  a  foot  in  width,  and  extends  westerly  into  the  hill. 
The  rock  of  the  band  is  thinly  laminated,  of  a  light  grey  color, 
and  finely  granular  in  texture.  It  also  contains  some  mica  in 
brownish  scales  between  the  laminae.  In  addition  this  rock  has 
a  distinct  banding  parallel  to  the  lamination.  We  immediately 
recognize  it  as  a  micaceous  quartzite,  and  so  closely  does  it  resem- 
ble much  of  the  Carboniferous  quartzite  already  studied,  as  to 
leave  no  doubt  that  it  was  derived  from  that.  But  its  contact 
with  the  neighboring  crystalline  rock  is  a  contact  of  fusion.  Then 
here,  too,  we  are  forced  to  believe  that  this  fragment  of  quartzite 
became  included  in  the  crystalline  rock  when  the  latter  was  a 


60  GEOLOGY    OF  WORCESTER. 

molten  mass.  Here,  also,  the  crystalline  rock  was  apparently  more 
affected  by  the  contact,  for,  on  either  side  of  the  inclusion,  the 
rock  differs  in  appearance  from  the  normal  crystalline  rock.  There 
is  a  like  effect  where  the  crystalline  rock  and  quartzite  are  near 
to  each  other  in  East  Kendall  street — the  granite  suffered  the 
greater  modification. 

These  inclusions  of  phyllite  and  quartzite,  together  with  the 
other  facts  before  considered,  make  clear  to  us,  that  the  phyllite 
and  quartzite  were  in  this  region  before  the  crystalline  rock,  and 
are  then  older  in  a  geological  sense;  and  that  the  crystalline  rock 
was  in  a  molten  condition  when  it  came  into  its  present  position. 
This  crystalline  rock  is,  then,  since  it  is  composed  of  the  minerals 
already  described,  a  granite.  We  may  call  it  a  post-Carboniferous 
granite  in  the  geological  series. 

The  granite  The  shattering  of  the  rocks,  already  described  and 

of  Millstone  so  noticeable  about  Millstone  Hill,  may  be  made  use 
at' some 'depth  of  along  another  line  of  thought.  So  far  as  is  known, 
beneath  the  molten  rock  does  not  solidify  in  the  form  of  granite 
at  the  surface  of  the  earth,  but  in  the  various  forms 
of  lava  seen  in  connection  with  volcanic  activity.  For  molten  rock 
to  solidify  in  the  form  of  granite,  it  must  do  so  slowly,  giving  time 
for  the  various  minerals  to  form  and  crystallize,  and  these  condi- 
tions prevail  more  or  less  deeply  within  the  earth.  So  that,  when- 
ever we  see  granite  at  the  surface  of  the  earth,  we  rest  assured  that 
it  has  not  always  been  there,  but  is  now  seen  at  the  surface  because 
the  rocks  that  formerly  covered  it  have  been  removed.  In  looking 
at  the  granite  of  Millstone  Hill  we  may  be  sure  that  a  greater  or 
less  thickness  of  quartzite  and  phyllite  has  been  removed  from 
above  it,  so  that  we  may  build  up  the  whole  surface  of  the  earth 
around  to  a  considerable  height  above  Millstone  Hill,  if  we  would 
go  back  to  the  time  when  this  molten  rock  mass  came  into  its 
present  position.  It  then  becomes  an  interesting  problem  for  us, 
if  possibly  we  can  solve  it,  to  determine  the  depth  at  which  this 
granite  solidified.  The  shattering  of  the  neighboring  rocks  may 
help  us  along  this  line  of  thought. 

Let  us  think  of  a  cubic  foot  of  rock  one  hundred 

method  of        feet  beneath  the  earth's  surface.     It  is  evident  that 

measuring  the     tnjs  cubic  foot  of  rock  supports  the  column  of  rock 

depth  at 

which  the        above  it.     This  column  of  rock  is  pressing  down  on 
granite          j^      Qr  jf  fae  cubjc  foot  of  rock  is  one  thousand  feet 

solidified. 

beneath  the  surface,  it  is  under  still  greater  pressure. 


GEOLOGY   OF   WORCESTER.  61 

Whatever  its  depth,  it  is  under  the  pressure  due  to  the  superin- 
cumbent rock  mass.  This  pressure  may  become  so  great  as  to 
exceed  the  strength  of  the  rock — so  that  the  outside  pressure 
forces  the  particles  of  the  rock  mass  together  more  firmly  than 
does  the  cohesion  between  the  particles  hold  them  together.  Under 
these  conditions  a  rock  cannot  break  so  as  to  produce  fissures  and 
cracks,  the  external  pressure  would  cause  the  rock  mass  to  yield 
by  crumbling  or  crushing  or  flowing.  The  rock  particles  must 
move  one  on  another;  they  cannot  move  away  from  each  other 
as  they  do  in  forming  a  fissure  or  crack.  If  then  the  yielding  of 
any  rock  during  a  disturbance  produces  cracks  or  fissures,  it  is 
evident  that  the  rock  cannot  have  been  at  a  depth  so  great  that 
the  pressure  from  the  superincumbent  rock  mass  exceeded  its 
strength.  When  this  quartzite  around  Millstone  Hill  was  shattered, 
producing  those  many  fissures  and  cracks  now  filled  by  quartz 
veins,  it  was  not  under  pressure  greater  than  its  strength.  When 
the  graphite  deposit  was  shattered,  it  also  was  under  pressure  less 
than  its  strength.  When  also  the  phyllite,  now  inclosed  in  the 
granite,  was  broken  into  fragments  producing  a  breccia,  it,  too, 
was  under  pressure  less  than  its  strength.  These  rocks  are  not  all 
of  the  same  strength,  the  phyllite  and  graphite  are  much  weaker 
than  the  quartzite.  Evidently  these  rocks  could  not  have  been 
so  deep  in  the  earth  when  this  fracturing  took  place  that  the  pres- 
sure from  the  superincumbent  rocks  exceeded  the  strength  of  the 
weakest  of  the  shattered  rocks.  The  phyllite  is  probably  as  weak 
as  any  of  these  rocks,  and  being  included  in  the  granite,  promises 
to  give  us  the  most  definite  solution  of  the  problem.  Without  an 
exact  determination  of  its  strength,  it  is  impossible  to  tell  exactly 
how  deep  it  must  be  in  the  earth  to  have  the  outside  pressure 
exceed  its  strength — where  it  could  not  break  into  fragments.  We 
may  however  come  quite  near  to  this.  Prof.  Van  Hise J  says  in  this 
connection:  "In  the  case  of  a  soft  shale,  but  a  small  thickness 
of  superincumbent  strata,  possibly  500  meters  or  less,  may  pre- 
vent any  considerable  fractures  and  crevices  from  forming." 
While  the  phyllite  may  be  more  resisting  and  stronger  than  what 
is  meant  by  soft  shale,  still  if  we  multiply  the  five  hundred  by  two 
or  three,  the  depth  will  be,  at  the  most,  a  few  thousand  feet.  If 
then  we  think  of  the  granite  as  having  come  into  its  present  posi- 


Sixteenth  Annual  Report  U.  S.  G.  S.,  Part  1,  p.  589. 


62  GEOLOGY    OF   WORCESTER. 

tion  at  a  depth  not  exceeding  four  or  five  thousand  feet,  we  cannot 
be  far  from  the  truth.  That  this  conclusion  is  approximately 
correct  is  confirmed  by  other  considerations.  Many  granites  give 
evidence  of  having  been  subjected  to  great  pressure  after  solidifi- 
cation because  the  quartz  and  feldspar  particles  are  reduced  to 
fine,  granular  masses  as  the  result  of  crushing.  Along  with  this 
crushing  there  is  developed  more  or  less  of  a  foliated  structure 
within  the  granite,  because  of  a  mechanical  rearrangement  of  the 
mineral  particles.  In  this  granite  of  Millstone  Hill  there  is  neither 
the  granulation  from  crushing  nor  foliation  from  great  pressure, 
except  in  a  very  narrow  zone  bordering  some  trunk  joints  special- 
ly noticeable  in  the  cliff  in  the  eastern  part  of  Normal  School  Hill. 
The  local  crushing  and  foliation  there  seen  are  undoubtedly  due  to 
the  movement  of  these  huge  blocks  of  granite,  one  upon  another, 
and  the  effects  of  this  motion  are  only  superficial  and  do  not  in 
the  least  contradict  our  conclusion.  Another  consideration  that 
confirms  the  conclusion  in  regard  to  the  small  depth  of  this  granite 
beneath  the  earth's  surface  is  the  fact  that  early  in  the  history  of 
this  granite  huge  cracks  or  fissures,  as  we  shall  later  explain,  were 
formed  through  the  mass  of  the  granite.  These  were  formed  prob- 
ably soon  after  the  granite  had  solidified.  Now,  that  cracks  and 
fissures  should  form  through  the  granite,  indicates  that  it  was  not 
so  deep  within  the  earth  that  the  outside  pressure  from  the  super- 
incumbent rocks  was  sufficient  to  keep  these  fissures  from  forming. 
Hence  we  may  conclude  from  this,  that  this  granite  was  not  a  very 
deeply  buried  granite.  While  these  other  considerations  do  not 
indicate  with  definiteness  the  shallowness  of  this  granite  within 
the  earth,  they  confirm,  or  at  least  do  not  contradict,  the  conclu- 
sion drawn  from  the  brecciated  phyllite  found  within  the  granite. 
Thus  it  is,  by  the  attending  phenomena,  that  we  may  measure  the 
original  depth  of  this  granite,  and  determine  the  thickness  of  the 
rock  mass  since  removed  by  erosion. 

In  this  discussion  in  regard  to  the  relation  of  this 
in  granite        granite  of  Millstone  Hill  to  the  surrounding  rocks,  we 
of  Millstone      have  been  led  away  from  our  study  of  the  rock  of 
the  quarry.     Following  the  road  leading  along  the 
western  side  of  the  quarry,  as  we  approach  the  northern  part  of 
the  quarry,  we  notice  what  seems  to  be  a  high  wall  extending  ap- 
proximately northwest  by  southeast  almost  continuously  across 
the  quarry,  and  rising  to  different  heights  above  the  floor  of  the 


v 


PART  OF  THE  APLITE  DIKE  LEFT   STANDING  ABOVE  THE  NEIGHBORING 
KOCK  OF  THE  QUARRY,  MILLSTONE  HILL. 


GEOLOGY   OF   WORCESTER.  63 

quarry,  where  highest,  perhaps  twenty-five  or  thirty  feet.1  In 
width  it  also  varies,  at  the  southeast  end  being  about  thirteen 
feet  wide,  and  in  other  parts  seventeen  to  twenty.  We  see  that 
this  wall  is  what  the  workmen  have  left  in  quarrying,  and  its  upper 
surface  shows  us  the  former  level  of  the  rock  surface  in  the  top  of 
this  hill.  From  this  we  may  gain  an  idea  of  the  large  quantity 
of  rock  that  has  been  removed  from  this  hill  by  the  quarrymen. 
But  the  question  naturally  rises  why  the  workmen  left  this  wall 
while  removing  the  adjacent  rock  to  the  base,  leaving  a  vertical 
surface  on  either  side. 

On  examining  the  rock  of  the  wall  and  compar- 
ing it  with  the  rock  of  the  rest  of  the  quarry,  the 
answer  is  quickly  found.  It  is  frequently  lighter 
in  color,  being  nearly  white;  it  is  much  more  finely  granular 
in  texture,  and  is  cut  by  joints  or  cracks  so  that  it  breaks  into 
small,  angular  blocks.  It  was  left,  evidently,  because  it  was 
not  so  well  fitted  for  use  as  a  building  stone.  Let  us  examine 
this  rock  more  carefully  to  determine  of  what  minerals  it  is 
composed,  and  discover,  if  we  may,  what  may  be  its  relation 
to  the  granite  already  studied.  First  we  examine  a  specimen 
which  is  almost  white.  We  easily  pick  out  the  feldspar  by  the 
smooth,  shining,  porcelain-like  cleavage  surfaces  glistening  in  the 
sunlight.  We  examine  a  number  of  these  under  the  magnifying 
glass,  and,  now  and  then,  see  one  having  the  fine  straight  lines  indi- 
cating a  triclinic  feldspar.  We  also  recognize  the  quartz,  though 
in  finer  particles,  by  its  glassy  lustre  and  want  of  cleavage,  looking 
very  much  like  fine  pieces  of  broken  glass.  These  minerals  we 
recognize  as  the  essential  constituents  of  this  rock.  In  addition 
we  may  find,  now  and  then,  as  we  move  the  magnifying  glass 
around  in  our  examination,  a  little  cube  of  a  brassy  color  and 
metallic  lustre.  Iron  pyrites  is  also  distributed  in  this  rock.  The 
lighter  variety  of  this  rock  is  then  composed  of  feldspar  and  quartz, 
is  of  a  crystalline,  granular  texture,  and  is  perfectly  massive  with- 
out foliation  or  banding.  These  are  characteristics  of  the  rock 
called  aplite.  But  this  rock  has  another  phase  slightly  different. 
It  has  a  decided  grey  color.  Under  the  magnifying  glass  we  see 
that  this  color  is  due,  in  large  part,  to  the  abundance  of  smoky  or 
grey  quartz  and  feldspar,  and  in  part  to  another  mineral  in  fine 


Since  the  above  was  written  this  wall  has  been  largely  removed  for  road  material. 


64  GEOLOGY    OF    WORCESTER. 

black  particles,  probably  biotite.  We  see  that  this  aplite 
is  closely  related  to  the  granite  as  far  as  the  minerals 
constituting  it  are  concerned,  but  it  is  finer  in  texture.  Continu- 
ing our  study,  we  search  for  the  border  line  between  the  granite 
and  aplite,  and  near  the  northwestern  end  of  the  wall  find  a  place 
where  we  may  clearly  see  the  granite  and  aplite  meeting  or  in 
contact.  At  this  place  there  is  not  a  blending  of  one  into  the 
other,  but  a  well  defined  line  between  coarse  granite  on  one  side 
and  fine  aplite  on  the  other.  There  is  not  however  any  break  or 
crack  or  fissure  in  the  rock  along  this  line.  It  is  quite  evident 
that  this  union  or  joining  of  these  two  rocks  could  not  have  been 
made  when  both  were  solid.  It  is  clearly  a  contact  of  fusion.  It 
must  have  resulted  from  at  least  one  of  these  rocks  being  in  a 
molten  state  and  solidifying  on  the  other.  The  relation  of  these  two 
rocks  will  appear  more  clearly  if  we  now  study  rock  of  the  same 
kind,  as  it  occurs  in  the  southern  part  of  Millstone  Hill,  south  and 
east  of  Bell  Pond.  Here  the  aplite  frequently  occurs  in  the  midst 
of  the  granite,  in  bands  fifteen  feet  or  more  in  width,  and  extending 
in  various  directions.  On  the  east  side  of  the  pond,  extending 
down  under  the  water,  is  the  most  instructive  one  for  our  present 
study.  Here  we  may  follow  the  aplite  bordered  by  granite  on 
either  side  for  a  distance  of  twenty  feet  or  so,  in  a  northwesterly 
direction,  the  aplite  being  only  two  to  four  feet  wide.  As  we 
approach  the  edge  of  the  water  we  observe  a  branch  from  the 
aplite  extending  out  into  the  bordering  granite.  The  dividing  line 
between  the  granite  and  aplite  is  so  clearly  defined  that  we  may 
draw  a  pencil  along  it,  yet  there  is  no  fissure  or  crack.  This  narrow 
band  of  aplite  with  a  branch  extending  out  into  the  bordering 
rock  can  be  nothing  but  a  dike.  Reasoning  from  this  we  con- 
clude that  all  the  other  aplite  bands  are  also  dikes.  There  are 
many  of  these,  but  no  other  one  is  so  convenient  for  study  as  the 
large  one  up  in  the  quarry. 

These  dikes  have  an  interesting  story  to  tell  us  of 
what  has  taken  place  since  the  granite  came  into  its 
position,  and  solidified  beneath  the  upfolded  quart  zite 
and  phyllite.  After  the  solidification  of  the  granite  (but  how  long 
it  is  not  possible  to  tell,  though,  geologically  speaking,  probably 
only  a  short  time),  the  granite  was  broken  and  rent  by  fissures, 
some  fifteen  to  twenty  feet  wide,  others  but  a  few  feet  in  width, 
extending  in  various  directions  without  uniformity  as  far  as  has 


GEOLOGY   OF   WORCESTER.  65 

been  noticed.  These  fissures  were  deep  enough  to  reach  down  to 
molten  rock,  probably  a  part  of  the  granite  magma  that  had  not 
yet  solidified ;  and  up  into  these  fissures  flowed  more  molten  rock, 
completely  filling  them.  As  this  molten  rock  solidified,  it  became 
attached  to  the  granite  without  crack  or  fissure  between,  forming 
the  fusion  contact  now  seen.  By  cooling  more  rapidly  than  did 
the  granite,  it  came  to  have  a -finer  granular  texture, 
inclusion  of  Ijet  us  now  return  to  the  large  dike  in  the  northern 
phyiiite  in  part  of  the  quarry.  When  we  have  gone  about  one- 
third  of  its  length  from  the  northwestern  end,  where 
the  contact  of  the  aplite  and  granite  is  very  distinct,  we  find  a 
place  where  there  is,  in  the  midst  of  the  aplite,  and  separated  by 
it  from  the  granite,  a  thin  band  of  rock  quite  different  from  either 
granite  or  aplite.  This  rock  is  of  a  light  slate  color;  the  material 
of  it  is  arranged  in  thin  fissile  laminae;  it  has  a  smooth  feel,  and 
presents  in  places  a  finely  corrugated  or  wrinkled  surface.  In  fact 
this  rock  bears  a  close  resemblance  to  the  phyiiite  of  the  surround- 
ing area,  and  is  evidently  a  small  fragment  of  it  inclosed  in  the 
aplite.  But  we  may  rightly  ask  how  it  got  there.  We  may  think 
of  it  as  a  fragment  of  the  phyiiite  which  fell  into  this  fissure  as 
the  granite  was  rent  asunder,  and  became  surrounded  by  the  up- 
rising molten  aplite,  and  there  held,  as  the  rock  solidified,  until  the 
work  of  the  quarrymen  exposed  it  to  view.  This  included  phyiiite 
is  a  striking  confirmation  of  the  idea  already  expressed,  that  the 
phyiiite  once  extended  over  the  top  of  Millstone  Hill,  for,  unless 
this  were  true,  certainly  none  of  it  could  have  fallen  into  the  fis- 
sure. As  we  go  along  we  see  many  cracks  or  fine 
"theapute™  fissures  running  through  this  aplite  in  various  direc- 
tions. These  are  so  numerous  in  places  as  to  produce 
almost  a  network.  It  must  have  been  subjected  to  great  tension, 
perhaps  from  cooling,  at  some  period  which  has  produced  these. 
Many  of  these  have  since  been  filled  with  fine  glassy  quartz  which 
has  been  brought  in  and  deposited  there  by  water.  Frequently 
these  cracks  are  bordered  on  either  side  by  a  darker  grey  band. 
This  effect  was  evidently  produced  by  material,  also  brought  into 
these  cracks  by  water,  which  soaked  out  into  the  feldspar  and 
quartz,  coloring  them.  Because  of  this  coloring  the  cracks  are  very 
noticeable,  and  may  be  observed  in  other  dikes  in  various  parts 
of  the  hill. 

6 


66  GEOLOGY   OF   WORCESTER. 

As  we  approach  the  road  crossing  the  aplite  dike 
The  oxidized      near  ^ne  southeast  end.  another  fact  attracts  our  at- 

porous  border  . 

of  the  apiite.  tention.  In  the  south  part  of  the  dike  where  it  comes 
in  contact  with  the  granite  the  rock  is  peculiar.  It 
is  very  rusty,  and  is  not  compact,  but  made  up  of  thin  sections, 
bent  and  broken  and  separated  by  thin,  flat  cavities.  The  sur- 
faces of  these  cavities  are  covered  with  yellow  iron  rust.  Under 
the  magnifying  glass  we  find  these  thin  sections  to  consist  of  fine, 
granular,  glassy  quartz  of  sugary  whiteness.  Evidently  these  flat 
cavities  have  been  formed  by  the  dissolving  out  by  water  of  some 
mineral,  which  was  evidently  oxidized  as  it  was  dissolved,  and  the 
iron  in  it  became  iron  rust  forming  a  lining  to  the  cavity.  What 
this  mineral  was  has  not  been  determined,  as  this  dissolving  out 
and  oxidation  have  gone  as  deeply  as  we  are  able  to  dig  into  the 
rock.  We  can  only  conjecture  as  to  what  it  was.  It  may  have 
been  a  carbonate  containing  considerable  iron;  and  this  is  quite 
probable,  as  in  another  part  of  the  quarry  we  find  such  a  carbonate 
at  the  present  time  in  the  process  of  solution  and  oxidation.  Pos- 
sibly the  mineral  was  a  sulphide  of  zinc  containing  much  iron,  for 
we  find  such  a  sulphide  in  other  parts  of  the  quarry. 

But  we  return  to  the  study  of  the  rock  itself.  By 
breaking  out  many  pieces  we  find  a  part  where  this 
white,  sugary  quartzite  is  compact,  without  cavities, 
and  is  in  contact  with  the  aplite.  Here  we  see  that  the  aplite  has 
attached  itself  to  this  quartzite  just  as  it  did  to  the  granite,  and 
so  we  are  led  to  believe  that  this  white  quartzite  is  a  fragment 
from  one  of  the  neighboring  rocks.  Possibly  it  is  a  fragment  from 
a  white  variety  of  the  carboniferous  quartzite,  or  possibly  it  is  a 
quartzite  fragment  that  was  included  in  the  granite  before  the 
aplite  filled  this  fissure.  Certain  it  is  that  this  quartzite  closely 
resembles  a  much  earlier  quartzite  found  at  the  surface  of  the 
earth  far  to  the  east,  but  which  probably  extends  under  this  area 
at  a  considerable  depth.  Possibly,  then,  it  is  a  fragment  from  that, 
brought  up  in  the  molten  granite  as  the  magma  rose  through  the 
underlying  rocks. 

If  we  are  careful  here  in  our  search,  and  break  out 
breccia          many  fragments  for  examination,  we  shall  find,  be- 
tween this  rusty  quartzite  and  the  compact,  finely 
grained  aplite,  still  another  rock  which  at  first  seems  to  be  a  con- 
fused mixture  of  the  two.     On  closer  examination,  however,  we 


APLITE  BRECCIA    FROM    MILLSTONE    HILL.     ORIGINAL,  7  BY  5V2  INCHES. 
LIGHT  AREAS  ARE  APLITE  FRAGMENTS. 


GEOLOGY   OF   WORCESTER.  67 

are  able  to  arrange  the  aplite  by  itself  and  the  quartzite  by  itself. 
The  aplite  is  in  angular  fragments  or  blocks,  frequently  an  inch 
or  two  through.  Between  these  blocks  is  fine  grained,  white  or 
rusty,  sugary  quartz  resembling  that  of  the  quartzite  just  des- 
cribed. We  immediately  recognize  the  rock  as  a  breccia,  for  it  is 
made  up  of  angular  fragments  cemented  together.  We  can  see 
from  this,  that  in  this  limited  region,  in  some  way,  during  the 
vicissitudes  through  which  this  rock  mass  has  passed,  the  aplite 
here  was  broken  into  fragments,  and  then  these  fragments  were 
cemented  together  by  the  granular  quartz.  But  we  must  acknowl- 
edge that  this  is  but  the  beginning  of  the  complete  explanation 
which  we  are  not  now  able  to  give. 

Continuing  our  study  of  this  dike,  we  cross  the 
mK     quarry  road  to  where  the  dike  meets  the  eastern  side 

of  the  quarry.  Here  we  break  off  a  specimen  for 
examination,  and  immediately  notice  a  strong,  offensive  odor.  We 
break  off  another  piece  at  a  different  place,  and  again  the  odor  is 
perceived.  After  trying  the  rock  in  several  places  with  the  same 
result,  we  conclude  that  the  odor  belongs  to  the  rock  here,  at  least, 
for  we  have  not  observed  it  elsewhere  in  the  aplite.  To  what  this 
odor  is  due  we  do  not  know,  only  that  we  have  perceived  this  odor 
coming  from  other  rocks  when  broken,  especially  from  quartz; 
and  it  is  possible  to  find  smoky  quartz  in  this  quarry  affording, 
when  broken,  this  same  odor.  Because  of  the  odor  such  quartz 
is  called  fetid  quartz.  The  aplite  possessing  this  odor  may  be 
called  fetid  aplite. 

Beyond  the  clearing  of  the  quarry  it  is  not  possible  to  trace  this 
dike,  and  so  we  do  not  know  exactly  how  far  it  extends.  Very 
likely  it  extends  to  the  eastern  side  of  the  hill,  and  future  obser- 
vations may  reveal  it,  as  the  covering  is  removed. 

Having  followed  the  aplite  dike  across  the  quarry, 
e       we  next  seek  other  points  of  interest  presented  here. 

Near  the  extreme  northern  end  of  the  quarry,  on  the 
right  hand  side,  as  we  face  the  north,  may  be  found  a  small  area, 
a  foot  or  two  in  diameter,  marked  by  a  green  color.  It  is  clearly 
different  from  the  surrounding  rock,  and  it  is  evidently  a  crystal- 
line mass.  The  crystals  run  together  so  that  we  cannot  obtain 
individual  crystals.  The  mineral  is  not  clear  and  transparent,  only 
translucent.  It  is  very  hard.  Fine  fragments  may  be  picked  up, 
and  now  and  then  one  will  show  more  or  less  clearly  a  six-sided 


68  GEOLOGY   OF   WORCESTER. 

prism,  and  tell  us  that  this  mineral  is  beryl.  At  another  point  a 
little  farther  south  in  the  quarry  wall  occurs  a  similar  area  of  beryl. 
In  like  manner,  at  other  points,  probably,  this  mineral  may  be 
found  distributed  in  these  isolated  masses  within  the  granite.  These 
indicate  that  about  these  points  the  beryl,  which  was  distributed 
through  the  molten  magma,  crystallized,  and  must  have  crystal- 
lized before  the  other  minerals,  because  the  other  minerals  are  not 
included  within  the  beryl  mass. 

On  the  left  hand  side  of  the  northernmost  point  of  the  quarry 
is  another  locality  quite  rich  in  minerals.  The  area  is  small,  being 
only  four  to  six  feet  in  diameter,  and  the  weathered  surface  is 
covered  by  much  iron  rust  from  the  minerals  within.  On  break- 
ing into  the  rock  we  find  it  having  little  resemblance  to  the  average 
granite.  It  is  of  various  colors,  depending  on  the  abundance  of 
this  or  that  mineral,  and  frequently  the  rock  is  composed  largely 
of  a  light  colored  mica,  the  mica  scales  having  no  uniformity  of 
position.  The  minerals  are  generally  in  too  fine  particles  to  be 
studied  by  the  unaided  eye.  Looking  through  the  magnifying  glass 
we  instantly  recognize  six-sided  clear,  glassy  prisms 
of  a  green  color.  They  are  perfect  and  beautiful 
enough  to  be  used  as  gems,  if  only  they  were  larger 
and  could  be  separated  from  the  inclosing  rock.  They  are  crystals 
of  beryl,  and  are  much  more  satisfactory  than  the  beryl  occurring 
in  coarser  masses. 

There  is  also  here,  perhaps  more  abundantly  than 
the  beryl,  a  brownish,  amber  colored  mineral  having 
also  at  times  a  pinkish  tinge.  It  is  distributed  in 
grains  and  frequently  makes  up  a  considerable  part  of  the  rock, 
giving  its  color  to  the  whole.  Upon  testing  this  mineral,  it  proves 
to  be  a  garnet,  but,  unfortunately,  is  seldom  in  well  defined  crystals. 
By  searching  we  find  one  having  the  shape  of  the  rhombic  dode- 
cahedron. 

Still  another  mineral  observed  here  is  of  a  black 

sphalerite        color,  resinous  lustre,  and  presents  bright  cleavage 

surfaces  when  broken.     This  gives  tests  for  sulphur, 

iron,  zinc  and  manganese,  and  is,  then,  an  impure  sphalerite  or 

zinc  blende.     Because  of  its  impurities  it  is  more  interesting  than 

the  pure  would  be. 

By  searching  carefully,  especially  in  that  part  of 
the  rock  decidedly  micaceous,  we  may  find  a  mineral 
of  a  dark,  bluish  grey  color,  of  a  bright  metallic 


GEOLOGY   OF   WORCESTER.  69 

lustre,  distributed  in  small  thin  masses  which  cleave  into  thinner 
scales.  This  mineral  is  very  soft,  and,  drawn  across  the  page, 
leaves  a  mark  on  the  paper  resembling  that  of  the  pencil.  The 
mark  of  the  former  is  brighter,  especially  in  the  sunlight,  and,  on 
closer  examination,  may  be  seen  to  have  a  slight  greenish  tinge. 
On  testing  it  in  the  laboratory,  it  gives  a  sulphur  reaction,  clearly 
distinguishing  it  from  graphite.  It  is  the  mineral  molybdenite. 
Iron  pyrites,  known  by  its  brassy  color  and  hardness,  is  also  found 
here.  In  fact  it  occurs  so  frequently,  here  and  there,  through  the 
quarry  as  to  seem  hardly  worthy  of  mention. 

But  the  mineral  most  eagerly  sought  here,  which 

gives  the  greatest  pleasure  when  found,  at  least  to  the 

beginner,   is  fluorite  or  fluor  spar.     This  is  easily 

recognized  by  its  purple  or  amethystine  color.     It  is  here  distributed 

in  little  masses  within  the  rock.     Its   occurrence  here  is  similar 

to  its  occurrence  in  the  granite  proper,  of  which  we  have  already 

spoken,  though  here  it  is  in  much  larger  particles. 

The  area  of  Within  the  area  of  a  few  square  feet  are  found  all 

these  min-       these  minerals  crowded  together,  while  in  other  parts 

o^segregatton     of  tne  quany  we  maY  search  over  large  areas  without 

in  the          finding  even  one  of  them  so  abundantly.     This  area 

is  not  a  vein  in  which  water  has  deposited  these 
minerals.  As  far  as  we  can  see  the  rock  in  which  these  minerals 
are  is  an  original  part  of  the  granite  mass,  blending  as  it  does  in 
all  directions  into  the  ordinary  granite  of  the  hill.  This  mineral 
region  has  evidently  been  here  as  long  as  the  granite  has,  and  is 
in  no  sense  secondary  to  it.  But  the  explanation  of  this  area  is 
closely  connected  with  the  explanation  of  other  facts  before  noticed, 
and  we  will  attempt  to  explain  all  together.  We  have  considered 
the  evidence  that  leads  us  to  believe  that  the  granite  of  Millstone 
Hill  was  once  a  molten  mass,  and  in  that  condition  came  into  its 
present  position.  From  this  molten  mass  or  magma  various  minerals 
crystallized,  and  in  some  regions  of  the  granite  minerals  quite 
different  from  the  ordinary  minerals  separated  out.  We  may  in- 
quire in  what  condition  these  minerals  of  the  granite  must  have 
been  and  what  relation  they  bore  to  each  other  in  the  molten  mass. 
Rock  magma  We  mav>  °f  course,  think  of  this  molten  magma 
a  complex  as  a  mixture  of  various  molten  minerals, — so  much 

quartz,  so  much  feldspar,  so  much  mica,  and  so  on, 
each  mineral  retaining  its    identity  even  in  the  molten  mass.     A 


70  GEOLOGY   OF  WORCESTER. 

little  thought,  however,  will  probably  lead  us  to  another  conclusion. 
Temperature  has  much  to  do  with  chemical  action  between  dif- 
ferent substances  and  elements,  and  also  with  the  solution  of  sub- 
stances in  solvents.  Certain  elements  unite  at  high  temperature 
which  will  not  unite  at  the  ordinary  temperature.  A  common 
illustration  of  this  principle  is  seen  in  the  action  of  charcoal  and 
oxygen.  At  the  ordinary  temperature  a  piece  of  charcoal  may 
remain  indefinitely  in  contact  with  oxygen  of  the  air  without  any 
action  taking  place  between  them ;  but  at  a  red  heat  charcoal  and 
oxygen  readily  unite.  The  charcoal  then  burns.  Likewise  a  piece 
of  soft  iron  and  charcoal  may  remain  in  contact  at  the  ordinary 
temperature  indefinitely  without  action,  but  at  an  elevated  tem- 
perature the  carbon  is  absorbed  or  dissolved  by  the  molten  iron. 
The  melted  iron  is  capable  of  forming  a  chemical  compound  or 
compounds  with  the  carbon  of  the  charcoal.  The  iron  which  has 
thus  dissolved  carbon  is  quite  a  different  substance  from  the 
same  iron  without  the  carbon.  Allow  the  molten  iron,  saturated 
with  carbon,  to  cool  rapidly,  and  this  carbon  remains  united  with 
a  part  of  the  iron,  the  whole  forming  a  solid  solution;  but  allow 
the  same  iron  to  cool  slowly,  and  the  carbon,  in  part,  crystallizes 
out  from  the  iron,  and  appears  in  little  crystals  of  graphite  dis- 
seminated through  the  iron.  But  even  in  this  case  a  part  of  the 
carbon  remains  united  with  the  iron,  causing  the  cast  iron  to  be 
quite  different  from  the  soft  iron  without  any  carbon.  In  other 
words,  some  compounds  and  solutions,  formed  at  elevated  tempera- 
tures, may,  by  rapid  cooling,  be  preserved  at  the  ordinary  tem- 
perature; and  on  the  other  hand,  by  slow  cooling,  may  be  resolved 
into  quite  different  substances  by  a  rearrangement  of  the  elements 
and  substances  of  the  molten  mass. 

Effect  of  The  same  principle  holds  in  the  case  of  rocks.     The 

slow  cooling  molten  volcanic  rock,  cooling  rapidly,  forms  a  non- 
magma,  crystalline  glass.  This  possibly  preserves  the  relation 
which  the  different  substances  bore  to  each  other  in  the  molten 
mass.  This  apparently  non-crystalline  mass  is  really  a  solid  solu- 
tion consisting  of  minute  crystals  that  began  to  form  in  spite  of  the 
rapid  cooling;  of  substances  that  were  in  solution  and  solidified  in 
the  midst  of  their  solvent  because  not  allowed  time  to  separate  and 
crystallize  by  themselves;  and  of  the  solvent  that  held  these  sub- 
stances in  solution.  If,  on  the  other  hand,  the  molten  rock  mass 
that  solidified  into  glass  had  solidified  slowly,  allowing  complex 


GEOLOGY    OF   WORCESTER.  71 

substances  to  break  into  simpler  ones,  and  substances  in  solution  to 
separate  from  their  solvents,  and  then  the  solvent  or  solvents  them- 
selves to  crystallize,  the  cooled  rock  mass  would  have  become  a 
wholly  crystalline  rock  instead  of  a  non-crystalline  glass. 

Effect  of  But  temperature  is  not  the  only  agency  that  may 

pressure  on  influence  chemical  action  and  the  solution  of  sub- 
magma,  stances  in  solvents.  Pressure,  more  or  less  great, 
will  force  certain  elements  into  chemical  union  which  would  not 
unite  by  simple  contact  under  the  ordinary  pressure,  and  will 
force  substances  into  solution  which  would  not  otherwise  dissolve. 
In  like  manner  pressure  will  keep  together  the  elements  of  certain 
compounds  and  will  keep  in  solution  substances  which  would 
separate  under  the  ordinary  pressure.  In  other  words  complex 
compounds  and  solutions  may  exist  under  considerable  pressure, 
which  would  break  up,  or  tend  to  break  up,  under  reduced  pressure. 

crystam-  Ijfit  us  now  aPP^  tnese  principles  to  the  molten 

zation  of  the  mass  of  rock  which  finally  became  the  granite  of 
Millstone  Hill.  It  was  at  a  temperature  more  or  less 
high  so  that  all  the  minerals  within  its  mass  formed  one  com- 
plex solution;  but  this  was  not  at  the  surface  of  the  earth,  it 
was  thousands  of  feet  beneath  the  surface,  at  least  before  it  rose 
into  its  present  position.  Being  thus  deep  within  the  earth,  it 
must  have  been  under  correspondingly  great  pressure.  Under  this 
pressure  a  complex  solution  might  be  possible  that  would  not 
be  possible  under  less  pressure  with  the  other  conditions  the  same. 
Then  as  the  molten  rock  mass  rose  towards  the  surface  of  the  earth, 
the  pressure  decreased  more  or  less  slowly,  depending  on  the  rate 
of  the  upward  motion.  With  this  decrease  of  pressure  we  may 
think  of  a  slow  cooling  of  the  molten  mass  taking  place — conditions 
which  would  be  favorable  to  the  beginning  of  the  separation  and 
crystallization  of  simpler  minerals.  This  crystallization  took  place 
in  the  midst  of  a  liquid  where  there  was  nothing  to  interfere 
with  the  formation  and  growth  of  perfect  crystals,  and  so  these 
first  minerals  formed  in  well  defined  crystals. 

Moreover,  as  this  crystallization  took  place,  the  remaining  mol- 
ten magma  became  simpler  by  the  amount  of  substance  removed, 
and  was  then  capable  of  holding  together  under  the  changed  condi- 
tions of  temperature  and  pressure,  and  so  the  crystallization  stopped, 
only  to  begin  again  when  the  conditions  of  temperature  and  pres- 
sure again  changed  so  that  this  still  complex  solution  of  the 


72  GKOLOGY   OF  WORCESTER. 

magma  could  not  hold  together.  When  the  final  crystallization 
took  place,  as  the  result  of  the  slow  cooling  of  the  molten  magma 
inclosed  in  the  cooler  surrounding  strata,  then  the  crystals  and 
crystalline  aggregates,  before  formed,  were  inclosed,  here  and 
there,  wherever  they  happened  to  be,  in  the  solidified  mass. 

Thus  may  we  explain  the  occurrence  of  quartz  crystals  which 
may  be  found  in  the  mass  of  the  granite,  and  the  beryl,  and 
garnet  crystals  distributed  through  the  granite,  either  in  single 
crystals  or  in  crystalline  aggregates.  They  were  the  first  sub- 
stances to  crystallize  out  in  the  molten  magma,  and  were  formed 
of  material  which  could  not,  in  the  changed  conditions  of  the 
magma  rising  through  cool  strata  to  regions  of  less  pressure, 
remain  in  solution  in  the  complex  molten  substance. 

There  is  still  another  mineral  region  in  the  southern 
AnranUein  Par*  °^  *ne  °iuarry-  You  will  be  assisted  in  finding 
it  by  the  fragments  of  rock  lying  about  having  a 
resemblance  to  cinder.  These  fragments  are  dark  in  color,  very 
porous,  and  the  cavities  are  lined  with  a  black  coating.  On  break- 
ing into  one  of  these  you  may  find  the  undecayed  rock,  and  this 
will  assist  you  in  finding  the  same  rock  in  the  ledge  near  by.  There 
will  be  little  difficulty  in  finding  it  in  the  ledge,  as  it  occurs  in  quite 
a  large  area.  You  will  then  see  that  the  mineral,  which  so  easily 
decays,  first  turns  to  a  rusty  color,  as  it  begins  to  be  acted  on 
by  the  agents  of  the  air,  forming  brown  blotches  on  the  sur- 
face of  the  rock.  Within  the  undecayed  rock  you  will  find  this 
mineral  in  crystalline,  granular  masses,  varying  in  color  from  dark 
bluish  grey  to  pure  white.  It  is  soft,  and  effervesces  with  hydro- 
chloric acid,  showing  that  it  is  a  carbonate.  It  contains,  as  is 
shown  by  analysis, l  85.13%  CaCO3.  It  is  not  the  simple  carbon- 
ate of  calcium;  it  also  contains  iron  and  manganese  carbonates. 
We  may  think  of  it  as  calcite  in  which  a  considerable  part 
of  the  calcium  is  replaced  by  iron  and  manganese.  It  may  be 
the  mineral  ankerite.  From  this  it  appears  that  manganese  is 
quite  widely  distributed  through  the  granite  of  this  hill.  This 
mineral  area  does  not  have  the  appearance  of  a  vein  but  simply 
a  region  within  the  granite.  The  granite  probably  derived  this 
carbonate  from  the  phyllite  or  the  quartzite  when,  as  a  molten 
mass,  it  rose  and  came  in  contact  with  them,  for  in  them,  as  we 
have  already  pointed  out,  there  is  a  similar  carbonate. 

>  Analysis  by  Harold  Lane. 


GEOLOGY   OF   WORCESTER.  73 

vein  minerals  In  addition  to  these  areas,  minerals  may  also  be 
Quartz  crys-  found  in  veins  within  the  granite.  At  some  time 
there  were  formed  cracks  or  fissures;  through  these 
water  has  soaked  during  long  periods  of  time,  bringing  in  and  de- 
positing various  minerals.  In  such  veins  may  be  found  quartz 
crystals.  These  have  been  noticed  especially  on  the  south  surface 
of  the  large  aplite  wall  near  the  western  side  of  the  quarry.  These 
crystals  are  sometimes  glassy,  and  sometimes  milky,  quartz,  and 
show  the  six-sided  prisms  terminated  by  the  six-sided  pyramids. 
We  have  been  told  that  in  some  parts  of  this  hill,  while  digging 
cellars,  workmen  have  found  quite  thick  veins  of  quartz  contain- 
ing large  crystals,  and  the  workmen  supposed  these  were  diamonds 
and  filled  their  dinner  pails  with  them.  These  quartz  crystals 
indicate  the  action  of  hot  water  bringing  in  and  depositing  quartz 
in  the  fissures  of  the  rock. 

Green  pur-  ®n  ^ne  surface  °f  this  same  wall  may  also  be  found 

pie,  white        fluor  spar,  green,  purple  or  amethystine,  and  color- 

jrspar.        jegg  Qr  w^j^-e      gu^.  ^g  fluor  Spar  js  by  no  means 

limited  to  any  one  part  of  the  quarry.  Wherever  the  men  are  at 
work,  especially  in  the  deeper  parts  of  the  quarry,  the  blocks 
of  rock  will  frequently  be  found  coated,  to  a  greater  or  less 
extent,  on  surfaces  adjoining  fissures,  with  a  very  thin  layer  of 
the  purple  fluor  spar.  Nor  is  it  difficult  to  explain  whence  this 
fluor  spar  comes.  In  describing  the  granite  we  pointed  out  little 
particles  of  fluor  spar  distributed  through  its  mass.  These  parti- 
cles are  dissolved  by  waters  soaking  through  the  granite,  and  the 
fluor  spar  from  these  is  carried  into  the  cracks  and  fissures,  and 
there  deposited  as  a  coating  on  the  sides.  Through  the  agency 
of  the  soaking  waters  this  mineral  is  thus  concentrated,  and  hence 
occurs  in  much  larger  masses  in  these  veins. 

iron  Along  with  the  fluor  spar,  iron  pyrites  occurs  in 

pyrites.         fafa  masses  constituting  a  part  of  the  veins.     It,  too, 
has  been  concentrated  through  the  agency  of  water. 

Dark  grey  At  the  beginning  of  the  discussion  in  regard  to 

variety  of  the  Millstone  Hill,  a  description  of  the  normal  granite 
was  given.  That  description  does  not  apply  to  all 
of  the  granite.  In  some  areas  the  granite  is  of  a  dark  grey,  instead 
of  a  light  grey,  color.  This  is  due  to  the  dark  color  of  both  the 
quartz  and  much  of  the  feldspar.  This  quartz  is  of  a  very  dark 
smoky  color,  not  infrequently  black,  while  much  of  the  feldspar 


74  GEOLOGY    OF   WORCESTER. 

is  also  dark  grey.  Judging  from  the  number  of  striated  surfaces, 
there  is  no  more  of  the  triclinic  feldspar  in  this  dark  feldspar,  than 
there  is  in  the  white,  so  that  we  cannot  attribute  the  change  of 
color  to  a  change  in  the  feldspar.  On  studying  the  occurrence  of 
this  dark  granite,  we  notice  that  it  is  most  frequently  found  either 
bordering  the  phyllite  inclusions  or  bordering  cracks  and  fissures. 
This  observation  leads  us  to  suspect  that  the  darkening  of  the 
granite  is  due  to  the  taking  up  of  material  from  the  phyllite.  This 
took  place,  partly,  directly  from  the  phyllite  inclusions,  and  partly 
from  waters  that  had  been  in  contact  with  the  phyllite  and  had 
become  impregnated  with  matter  from  it,  which  soaked  through 
the  cracks  of  the  granite  and  imparted  this  matter  to  the  granite, 
and  thus  colored  it. 

In  the  southern  part  of  the  quarry,  south  and 


highly  impreg-     southeast  of  the  carbonate  area,  is  a  rock  of  pecu- 

nated  with 

solutions  from  har  appearance.  The  area  in  which  it  occurs  is  about 
the  granite.  eighi  feet  jn  width,  and  may  be  traced  fifty  feet  or 
more  in  an  easterly  direction.  The  rock  is  of  a  dark  grey  color, 
and  shows  a  trace  of  foliation  producing  a  cleavage.  On  such 
cleavage  surfaces  the  glassy  quartz  appears  in  black,  shining  par- 
ticles, entirely  distinct  in  the  inclosing  grey  mass.  These  quartz 
particles  are  not  crystals,  but  irregular,  rounded  masses  an  eighth 
of  an  inch  or  so  in  diameter.  So  distinct  are  these  from  the  in- 
closing material  that  they  produce  somewhat  of  a  porphyritic 
appearance.  After  much  thought,  and  the  examination  of  many 
specimens  from  this  area,  it  seems  reasonable  to  consider  this  rock 
as  a  phyllite  inclusion  in  which  the  phyllite  and  granite  have  af- 
fected each  other  to  a  much  greater  degree  than  they  did  in  the 
case  of  the  other  inclusion  described  in  an  earlier  part  of  this  chapter. 
Hot  liquids,  abounding  in  mineral  matter,  especially  quartz,  derived 
from  the  granite,  soaked  into  the  phyllite,  and  there  the  hot  solution 
deposited  minerals,  changing  the  phyllite  almost  beyond  recognition. 
On  the  other  hand,  also,  matter  seems  to  have  been  carried  from 
the  phyllite  into  the  granite  magma,  giving  to  the  feldspar  and 
quartz  a  color  as  dark  as  that  of  the  phyllite  itself.  Why  one 
inclusion  of  phyllite  should  be  so  changed,  and  another,  a  few  hundred 
yards  away,  remain  almost  unchanged,  is  difficult  to  explain.  The 
thickness  of  the  unchanged  one  will  not  explain  it,  as  this  is  but 
a  few  inches  thick.  Possibly  the  unchanged  one  became  inclosed 
in  the  granite  magma  when  dryer,  or  at  a  later  time,  when  the  magma 


A  WALL  OF  THE  QUARRY  IN  THE  TOP  OF  MILLSTONE  HILL,  SHOWING  THE 
JOINTING  OF  THE  GRANITE. 


GEOLOGY    OF   WORCESTER.  75 

was  somewhat  cooler  and  approaching  the  temperature  of  crystal- 
lization. But  even  in  this  case  the  granite  on  either  side  has  the 
dark  color,  showing  that  the  phyllite  affected  it,  if  the  granite  did 
not  affect  the  phyllite. 

There  is  another  fact  that  has  attracted  our  atten- 
^on  &s  we  nave  gone  about  the  quarry.  The  granite 
is  not  in  one  unbroken  mass,  but  divided  into  slabs 
of  varying  thickness,  which  are  approximately  parallel  in  position, 
thus  constituting,  as  it  were,  sheets,  one  above  the  other.  These 
sheets  do  not  extend  horizontally  across  the  hill,  but  curve  so  as  to 
be  parallel  with  the  outline  of  the  hill.  In  the  northern  part  of  the 
quarry  these  sheets  slope  down  to  the  northwest  at  an  angle  of 
about  twenty  degrees.  In  the  central  part  of  the  quarry  the  sheets 
are  approximately  horizontal;  and  in  the  eastern  part,  they  slope 
down  to  the  southeast  at  an  angle  of  about  twenty  degrees.  These 
sheets  are  seen  to  the  best  advantage  in  the  vertical  walls  left  by 
the  quarrying  away  of  the  rock. 

These  sheets  are  not  of  uniform  thickness.  At  the  top  of  the 
wall,  bounding  the  quarry  on  the  northeast  and  east  sides,  these 
sheets  are  not  more  than  six  inches  thick,  perhaps  even  less;  and 
they  seem  to  rapidly  increase  in  thickness  so  that  at  a  depth  of 
twenty-four  feet  they  are  apparently  two  feet  thick. .  But  on  care- 
ful study  we  see  that  this  thickening  is  largely  only  apparent. 
Near  the  surface  of  the  hill  every  crack  and  fissure  is  distinctly 
brought  out  by  weathering  and  frost.  The  former  has  produced 
a  border  of  iron  rust  on  either  side  of  every  crack,  while  the  latter 
has  enlarged  and  opened  them  so  that  there  the  rock  appears  in 
quite  thin  slabs.  Farther  down  in  the  granite,  even  at  the  depth 
of  twenty  feet,  it  is  seen,  on  close  observation,  that  there,  too,  the 
granite  is  cut  by  many  cracks  which  have  not  yet  been  so  distinct- 
ly brought  out.  Because  of  this  we  cannot  determine  just  how 
great  the  increase  in  thickness  of  the  slabs  is.  The  breaking  of 
the  granite  into  these  sheets,  wrapping  over  the  hill,  indicates 
tension  within  the  mass,  and  probably  this  tension  was  produced 
by  the  constant  expansion  and  contraction  of  the  superficial  parts 
of  the  granite  due  to  the  heating  of  the  rock  by  the  sun's  rays  and 
the  cooling  when  these  are  withdrawn. 

These  sheets  are  not,  however,  continuous,  but  are  crossed  by 
many  cracks  and  fissures  at  various  angles,  and  are  thus  cut  into 
slabs.  These  cross  fissures  are  more  or  less  nearly  at  right  angles 


76  GEOLOGY   OF  WORCESTER. 

to  the  sheets  and  may  be  divided  into  two  classes.  The  major, 
or  trunk,  fissures  and  the  minor.  The  minor  are  frequently  but  a 
few  feet  in  length  and  extend  in  any  direction.  The  surfaces  of 
the  granite  bordering  these  are  frequently  covered  by  a  thin,  shin- 
ing coating  of  fine  greenish  mica.  Because  of  the  cross  cracks,  the 
granite  breaks  into  irregular  blocks;  and  a  wall  built  out  of  these 
untrimmed  blocks  looks  as  if  it  had  been  built  after  a  crazy  quilt 
pattern. 

crushing  and  ^he  maJor  or  trunk  fissures  run  down  into  the 
production  granite  and  are  nearly  vertical  in  the  quarry.  In 
"structure1  other  parts  of  the  hill  they  deviate  somewhat  from 
along  joint  the  vertical  direction.  At  times  they  are  parallel,  at 
planes.  other  times  not  parallel.  These  are  specially  marked 
in  the  vertical  cliff  east  of  the  Normal  School.  Here  they  strike 
about  northwest,  and  have  a  dip  of  about  fifty  degrees  to  the 
southwest.  Here  these  fissures  are  parallel.  These  major  or  trunk 
fissures  are  bordered  by  a  thicker  coating  of  brownish  green  mica, 
and  the  rock  surfaces  are  smooth  and  polished,  and  covered  with 
striations  or  rounded  scratches.  Frequently  also  the  rock  on 
either  side  of  these  fissures  has  a  marked  foliated  structure;  and 
the  granite  within  these  folia  is  finely  granulated,  the  quartz  and 
feldspar  being  reduced  to  a  powder.  All  these  facts  point  to  great 
pressure  within  the  granite  mass  by  which  the  rock  was  broken 
to  great  depth,  as  it  were,  into  great  cakes  or  blocks;  these  blocks 
rubbed  and  crushed  against  each  other  producing  mica-covered 
and  striated  surfaces,  frequently  also  grinding  the  very  rock  to  the 
depth  of  several  inches  on  either  side  to  a  fine  powder  and  arrang- 
ing the  powdered  rock  in  thin  folia  parallel  to  the  fissure.  As  we 
look  at  the  cliff  east  of  the  Normal  School  it  does  not  require  much 
imagination  to  see  that  the  granite  of  this  ledge  was  subjected 
to  great  pressure  from  the  southwest  until  the  whole  mass  was 
broken  into  these  massive  blocks,  and  these  moved,  each  against 
its  neighbor,  grinding  and  crushing  the  surfaces.  These  facts  tell 
us  that  this  granite  has  not  remained  quietly  inclosed  in  the 
surrounding  rocks  through  the  ages,  while  those  above  were  being 
removed  by  erosion,  finally  bringing  it  to  the  surface,  or  rather 
bringing  the  surface  of  the  earth  down  to  it. 

Another  fact  noticed,  as  we  study  the  rock  of 
Millstone  Hill,  which  we  should  consider  before 
leaving  the  subject,  is  the  appearance  presented  by 


CLIFF  OF  GRANITE  EAST  OF  THE  NORMAL  SCHOOL,  SHOWING  THE  JOINTS 

BY    WHICH    THE   GRANITE    IS   CUT    INTO    ANGULAR    BLOCKS. 


GEOLOGY   OF   WORCESTER.  '.'7 

the  rock  surfaces  long  exposed  to  the  air,  or  bordering  the  trunk 
fissures.  These  surfaces  are  uniformly  very  rusty,  and  frequently 
black  in  color.  On  breaking  into  the  rock  we  find  the  black  a 
surface  coating,  while  the  rusty  appearance  extends  more  or  less 
deeply  into  the  rock,  forming  a  zone  frequently  several  inches  in 
thickness.  The  rusty  color  we  instantly  recognize  as  that  of  iron 
rust.  Upon  testing  the  black  coating,  we  find  that  it  gives  a  de- 
cided test  for  manganese,  and  will  set  chlorine  free  from  hydro- 
chloric acid.  We  therefore  conclude  that  in  the  black  coating  there 
is  considerable  black  oxide  of  manganese  mixed  with  the  iron  rust. 
These  oxides  of  iron  and  manganese  result  from  the  decay  of  this 
granite.  For  ages  water  has  been  soaking  through  the  fissures 
carrying  in  solution  oxygen  and  carbonic  acid  from  the  air,  and 
organic  acids  from  decaying  vegetation;  this  water  has  soaked 
into  the  rock,  on  either  side  of  the  fissures,  between  the  minerals 
and  along  cleavage  planes;  on  these  minerals  the  oxygen  and  acids 
have  acted,  taking  various  elements  out  of  the  minerals  and  lead- 
ing to  the  formation  of  new  substances.  As  this  takes  place  the 
minerals  crumble,  more  or  less,  and  the  rock  is  weakened.  If  the 
rock  happens  to  be  within  the  reach  of  frost,  that  helps  on  the  work 
of  disintegration.  In  this  change,  iron,  which  occurs  in  minerals  in 
the  granite,  becomes  iron  oxide  or  iron  rust,  and  shows  by  its  color 
the  distance  into  the  rock  to  which  the  waters  have  carried  oxygen 
from  the  air.  By  such  changes  this  granite  slowly  crumbles  into 
loose  particles  of  clay  and  sand.  As  this  change  takes  place  we 
notice  a  variation  in  the  thickness  of  the  weathered  zone.  This 
zone  is  noticeably  thicker  at  the  corners,  and  the  corners  of  the 
inside  surface  of  the  weathered  zone  are  more  rounded  than  are 
the  outside  corners.  This  is  because  the  weathering,  extending  in 
from  one  surface,  has  met  that  extending  in  from  the  adjacent 
surface,  and  as  a  result  the  corners  become  less  and  less  angular 
as  weathered  zone  after  weathered  zone  is  broken  off.  In  this 
way  an  angular  block  of  granite  tends  to  become  spherical  or  at 
least  rounded. 

Disintegra-  But  this  granite  contains  an  element  of  weakness 

tion  due  to       no^  aiways  found  in  granites.     It  contains  as  we  have 

and  iron         pointed  out  little  particles  of  fluor  spar  quite  thickly 

pyrites.         distributed  through  its  masses,  as  well  as  constituting 

veins  in  fissures.     The  granite  also  contains  iron  pyrites.     As  the 

latter  is  oxidized,  it  either  forms  an  acid  sulphate  or  free  sulphuric 


78  GEOLOGY    OF   WORCESTER. 

acid.  Either  of  these,  coming  in  contact  with  the  fluor  spar,  acts 
on  it,  setting  free  hydrofluoric  acid.  The  latter  acts  forcibly  on 
the  rock  minerals  around  the  fluor  spar,  and  causes  them  to 
crumble.  That  this  action  does  take  place  we  were  led  to  believe 
from  the  study  of  one  specimen  in  particular  which  abounded  in 
fluor  spar  and  iron  pyrites.  The  surrounding  minerals  had  been 
acted  upon  by  some  agent  so  that  they  crumbled  easily,  while 
the  iron  pyrites  and  fluor  spar  were  in  part  fresh  and  unacted  on. 
We  will  now  summarize  the  leading  facts  brought 
out  in  our  study  of  the  granite  of  Millstone  Hill. 
As  a  molten  magma  this  rock  rose  from  some  greater  depth  in  the 
earth  into  the  midst  of  the  Carboniferous  phyllite  and  quartzite, 
either  greatly  disturbing  and  moving  them,  or  flowing  through 
breaks  and  under  folds  that  were  produced  by  other  causes.  But  this 
molten  rock  did  not  reach  the  surface  of  the  earth ;  it  rose  to  within 
a  few  thousand  feet  of  that  surface  and  there  crystallized — certain 
minerals  crystallizing  before  the  general  solidification  took  place. 
Later  this  granite  mass  was  profoundly  broken  so  that  great  fissures 
extended  through  it  in  various  directions.  These  fissures  were  chan- 
nels into  which  more  molten  rock  flowed  and  crystallized,  completely 
filling  them,  and  making  the  aplite  dikes.  By  the  agents  of  erosion 
the  superincumbent  rocks  were  slowly  removed,  and  the  surface 
of  the  earth  was  brought  down  to  the  buried  granite.  But  no 
sooner  was  the  granite  uncovered,  than  the  same  agents  began  to 
work  on  it,  pulverizing  it  and  washing  it  away  to  form  sand  and 
mud  to  be  deposited  as  sediments  by  water.  The  ice  of  the  Glacial 
Period  also  removed  an  appreciable  amount  from  its  mass,  and 
spread  bowlders  from  it  over  the  fields  to  the  south.  If  only  these, 
together  with  the  finer  rock-flour  which  resulted  from  the  crush- 
ing of  many  other  fragments  of  this  granite,  could  be  returned  to 
this  hill,  its  top  would  be  sensibly  elevated.  Hence  it  is  that  the 
original  granite  mass  exceeded,  considerably,  that  with  which  we 
are  now  acquainted.  But  we  must  remember  that  what  we  see, 
appearing  only  in  the  top  of  the  hill  and  not  even  reaching  to  the 
visible  base,  gives  us  but  a  small  idea  of  the  real  extent  of  this 
granite.  As  it  rose  from  beneath,  the  direction  of  its  greatest 
extension  is  down  into  the  earth.  It  is  impossible  to  tell  exactly 
how  far  it  extends  down,  but  we  risk  nothing  in  prophesying  that 
it  extends  to  a  great  depth,  and  that  its  length  and  width  are  small 
compared  with  its  depth. 


WALL  OF  THE  BALLAKD  QUARRY,  NEAR  QUINSIGAMOND,  SHOWING  HORI- 
ZONTAL JOINTING  AND  BANDING  RESULTING  FROM  ALTERNATION 
OF  SCHIST  AND  GRANITE. 


CHAPTER  IV. 

BOLTON  GNEISS. 

Let  us  bear  in  mind  that  the  subject  for  our  study  is  the  rock- 
floor  of  Worcester.  In  this  study  we  have  now  considered  that 
which  underlies  the  central  part  of  Worcester  from  the  northern 
to  the  southern  boundary.  There  remain  for  our  consideration 
the  extreme  eastern  and  western  areas  beyond  the  boundaries  of 
the  quartzite. 

That  we  may  begin  in  this  new  study  at  a  point 

Southeast  .  . 

slope  of  where  we  are  already  acquainted,  let  us  again  go  to 
wigwam  Wigwam  Hill.  Instead  of  studying  the  top  of  the 
hill,  let  us  go  down  the  southeastern  slope.  A  few 
hundred  feet  from  the  Carboniferous  quartzose  mica  schist,  we 
find  a  ledge  which  instantly  appeals  to  us  as  of  quite  different  rock. 
It  is  laminated  or  foliated,  but  we  look  in  vain  for  the  extreme 
folding  and  crumpling  so  noticeable  in  the  rock  at  the  top  of  the 
hill;  and  this  rock  is  not  traversed  by  any  such  bands  or  beds  of 
granular  quartz.  It  has  a  finely  speckled  appearance  due  to  white 
particles  inclosed  by  a  dirty,  dark  grey,  micaceous  mass.  It  points 
seventeen  degrees  east  of  north  and  dips  about  eighty  degrees  to 
the  west.  Breaking  off  a  piece  for  study,  we  find  the  rock  highly 
micaceous,  consisting  in  considerable  part  of  fine  scales  of  brown- 
ish mica  approximately  parallel  to  each  other.  On  a  surface  at 
right  angles  to  the  laminae  or  folia  we  see  that  the  white  spots 
referred  to  are  flattened  lenses,  sometimes  of  quartz  and  sometimes 
of  feldspar,  while  the  fine  white  material  is  probably  a  mixture  of 
much  quartz  with  a  little  feldspar.  This  rock  is  a  well  foliated 
gneiss.  From  here  the  formation  of  which  this  rock  is  a  part, 
may  be  traced  far  to  the  northeast,  beyond  the  bounds  of  Worces- 
ter; to  the  south  also  it  may  be  traced  many -miles,  lying,  as  it  does 
here,  just  east  of  the  Carboniferous  quartzite.  This  formation 
constitutes  the  rock-floor  in  the  extreme  eastern  part  of  Worcester. 
Moreover  it  is  not  limited  to  Worcester  in  its  eastward  extension, 
but  may  be  traced  by  the  ledges  many  miles  to  the  east,  as  well 


80  GEOLOGY   OF   WORCESTER. 

as  to  the  north  and  south,  beyond  Worcester.     It  is  of  great  extent 
here  in  Central  Massachusetts. 

Starting  then  from  Wigwam  Hill,  we  may  trace 

quarry  near       this  formation  by  means  of  the  outcrops  appearing  in 

the  fields,  down  to  the  quarry  belonging  to  the  Bal- 

lard  estate,  near  Quinsigamond.     Here,  because  of  the 

extensive  cut  that  has  been  made  in  it,  we  may  study  this  forma- 

tion to  the  best  advantage.     Here  everything  is  in  sharp  contrast 

with  what  was  seen  at  Millstone  Hill.     Everywhere  the  rock  is 

banded. 

The  light  and  dark  alternating  bands  are  almost  as  distinct  and 
clearly  cut  as  are  the  bands  in  agate.  We  may  trace  these  bands 
across  the  quarry,  except  where  they  are  concealed  by  the  quarry 
road  and  the  piles  of  quarried  rock.  These  bands  are  in  general 
parallel,  and  the  direction  is  approximately  the  same  throughout 
the  quarry,  except  where  there  is  a  local  bending  or  folding, 
throwing  the  bands  out  of  the  general  direction  for  a  short  distance. 
This  general  direction  is  twenty-five  to  thirty  degrees  east  of  north. 
These  bands  are  also  tilted  up  at  a  high  angle,  frequently  stand- 
ing almost  vertically,  having  a  dip  of  seventy  to  eighty-five  de- 
grees to  the  west. 

This  description  will  serve  to  give  us  a  general  idea  of  the  rock 
of  this  quarry,  but  there  are  many  interesting  facts  here  presented 
which  will  appear  only  with  more  careful  study.  Let  us  then  study 
the  separate  bands  appearing  in  the  floor  of  the  quarry,  beginning 
in  the  middle  of  the  west  side,  and  going  to  the  east.  We  shall 
thus  walk  across  the  edges  of  the  different  bands. 

(1)  The  first  band  is  about  twenty-seven  feet 
the'rock  of  wide.  The  rock  of  it  is  of  a  light  grey  color,  of 
the  quarry  coarsely  crystalline  texture,  and  of  foliated  structure. 
This  structure  is  due,  in  part,  to  the  distribution  of  the 


black  mica  in  thin,  discontinuous  sheets;  and,  in  part. 

to  the  uniform  arrangement  of  the  intervening  par- 
ticles so  that  their  longer  axes  are  parallel  to  the  mica  sheets.  Both 
particles  and  mica  sheets  are  parallel  to  the  general  banding  of  the 
quarry  rock.  This  arrangement  of  the  minerals  gives  to  this  rock  a 
streaked  appearance  rather  than  a  distinct  banding.  The  minerals, 
in  addition  to  the  mica,  making  up  this  rock  are  feldspar  and  quartz. 
The  former  is  recognized  by  its  many  cleavage  surfaces  and  its  por- 
celain-like lustre.  It  is  generally  pure  white  in  color,  but  has,  now 


GNEISSOID   GRANITE   FROM    BALLARD'S    QUARRY,   NEAR    QUINSIGAMOND. 
ORIGINAL,  5  INCHES  BY  4. 


GEOLOGY   OF   WORCESTER.  81 

and  then,  a  slight  greyish  tint.  The  feldspar  particles  vary  greatly 
in  size.  Some  are  half  an  inch  or  less  in  diameter,  others  are  three 
to  four  inches  long  by  one  to  two  inches  wide,  and  there  are  all 
the  intervening  sizes  between  these  two  extremes.  Within  these 
feldspar  particles  may  be  seen,  under  the  magnifying  glass,  little 
inclusions  of  quartz,  rounded  in  shape,  sometimes  oblong  and  some- 
times cylindrical  and  worm-like.  The  feldspar  particles  do  not 
show  the  definite  outline  of  individual  crystals,  but  join,  one  to 
another,  forming  a  confused  crystalline  mass.  Now  and  then  a 
feldspar  individual  is  separated  from  the  surrounding  minerals  by 
mica;  even  then  it  does  not  present  the  outline  of  a  crystal,  but 
is  flattened  and  rounded  into  the  shape  of  a  lens  lying  parallel  to 
the  banding  of  the  rock.  The  feldspar  does  not  present  striated 
surfaces,  and  so  we  conclude  that  it  is  all  orthoclase  feldspar.  As 
we  study  it  thus  carefully,  we  notice  that  the  lens-shaped  particles 
frequently  have  a  rim  of  finely  granular  feldspar,  the  latter  a  part 
of  the  rounded  particle,  yet  having  this  different  texture.  If  we 
think  of  this  particle  as  having  been  partially  crushed,  the  crush- 
ing reaching  in  but  a  short  distance  from  the  surface,  this  fine 
granular  rim  is  quite  easily  explained.  This  is  but  an  indication 
of  the  dynamic  changes  to  which  this  and  other  rocks  have  been 
subjected.  But,  as  we  are  studying  the  feldspar  under  the  glass, 
we  notice,  here  and  there,  inclosed  in  the  feldspar  little  amber 
colored  crystals.  They  are  in  the  shape  of  four-sided  prisms,  and 
shine  with  an  adamantine  lustre.  They  are  crystals  of  the  mineral 
zircon.  Though  they  are  in  this  rock  they  are  not  an  essential 
part  of  it;  they  are  simply,  as  it  were,  accidental.  This  rock  con- 
tains the  minerals  of  granite.  The  feldspar  is  abundant  and  the 
quartz  not  more  so  than  might  be  expected  in  granite;  yet  the 
rock  has  a  marked  foliated,  almost  banded  structure,  resulting 
from  the  arrangement  of  the  minerals.  The  latter  are  character- 
istics of  gneiss.  Because  of  the  resemblance  which  this  rock  bears 
to  both  granite  and  gneiss  it  may  be  called  a  gneissoid  granite. 
Hombiendic  (2)  Lying  adjacent  to,  and  just  east  of,  this  first 
mica  schist  band  is  a  very  different  rock.  This  is  five  feet  in 
or  gneiss.  wjdth.  It  consists  of  alternating  light  and  dark 
bands  varying  in  thickness  from  one-eighth  to  one-half  inch. 
The  dark  bands  are  characterized  by  an  abundance  of  dark 
brownish  mica.  The  fine  scales  of  the  mica  are  so  abundant  as 
to  conceal  quite  effectually  the  feldspar  and  quartz,  which  are 


82  GEOLOGY    OF   WORCESTER. 

also  present  in  fine  particles,  and  cannot  be  distinguished  even 
under  the  magnifying  glass.  The  lighter  bands  have  a  green- 
ish color  because  of  the  small  particles  of  light  green  hornblende 
distributed  quite  abundantly  through  them.  There  may  be  seen 
in  these  also  a  dark,  greenish  black  hornblende  occurring  in  small, 
blade-like  masses,  sometimes  a  half  inch  in  length.  On  closer 
examination  of  the  greenish  bands  under  the  magnifying  glass 
there  are  seen,  scattered  here  and  there,  little  amber  colored 
crystals.  We  cannot  make  out  their  crystalline  form,  but  think 
they  are  most  likely  little  amber  colored  garnets.  These  are  not, 
however,  an  essential  part  of  the  rock,  and  attract  our  attention 
by  their  color  before  other  minerals,  which  are  more  important  in 
helping  us  to  name  the  rock.  There  are  also  quartz  of  a  bluish, 
smoky  tint,  and  feldspar,  both  in  fine  particles. 

These  alternating  light  and  dark  bands  are  generally  perfectly 

distinct  so  that  a  pencil  may  be  drawn  along  the  dividing  line. 

The  rock  of  this  second  band  of  the  quarry,  because  of  its  banded 

structure  and  the  minerals  it  contains,  may  be  called  a  gneiss, 

though  the  more  micaceous,  small  bands  may,  by  themselves,  be 

called  mica  schist,  and  the  hornblendic  bands,  hornblende  gneiss. 

Gneissoid  (3)     Next  east  of  this  banded  gneiss  is  another 

granite.         band  twenty-four  feet  wide,  of  coarse,  light  colored 

gneissoid  granite  like  the  first  band. 

(4)     Then  east  of  this  is  found  a  band,  also  twenty - 
f°ur  ^eet  in  width.     It  consists  largely  of  a  dark  grey 
ametamorphic     biotite  or  black  mica  arranged  so  as  to  give  a  lami- 
"JJXr         nated  structure.     This  may  properly  be  called  biotite 
schist.     Imbedded  in  it  are  thin,  wide,  long  masses 
of  lighter  rock  which  on  cross-sections  appear  as  long,  wavy,  narrow 
bands  or  streamers.     This  lighter  rock  substance  is  mainly  white, 
coarse,  pearly  feldspar  with  fine  granular  quartz,  and  is  frequently 
divided  into  rounded  or  lens-shaped  masses  by  films  of  biotite, 
producing  a  resemblance  to  flattened  pebbles  in  a  metamorphic 
conglomerate. 

AISO  resem-          (5)     Next  is  a  band,  twenty-one  feet  wide,  of  rock 
bies  a  meta-      resembling  the  last,  but  coarser  in  texture,  in  which 

morphic    con- 

glomerate.  the  pebbly  character  is  even  more  clearly  brought  out. 

Hornblendic  (6)     Then  there  is  a  band,  thirty-five  feet  wide, 

biotite  gneiss  made  up  of  narrower  alternating  bands  of  biotite 

Il8t'  and  hornblende  gneiss,  brought  out  clearly  by  the 


A   SLAB  SHOWING  THE  ALTERNATING    LAMINAE    OF    HORNBLENDE    AND 
MICA  SCHISTS,  QTJINSIGAMOND  QUARRY. 


GEOLOGY    OF   WORCESTER.  83 

variation  in  color;  and  now  and  then  there  is  a  band  much  lighter 
in  color  than  the  average  of  either. 

(7)  Continuing  our  study  to  the  east,  we  next 
meet  a  rock  quite  different  in  appearance  from  any 
before  met.  It  is  of  a  light  grey  or  slightly  brown- 
ish grey  color;  and  consists  of  a  fine  grained  mixture  of  quartz, 
feldspar  and  mica.  These  minerals  are  arranged  so  as  to  give  a 
distinct  finely  banded  structure  parallel  to  the  strike  of  the  rock 
of  the  quarry.  These  fine  bands  are  about  one  sixteenth  of  an 
inch  thick,  and  are  grouped  together  so  as  to  give  a  coarser  band- 
ing; in  addition  there  are  other  bands  six  to  twelve  inches  thick, 
uniform  in  color  and  not  showing  any  bands  within  themselves, 
but  having  a  distinct  foliation  due  to  the  arrangement  of  the  min- 
erals. There  is  no  clearly  defined  line  between  the  light  grey 
which  shows  the  banding  of  a  gneiss  to  perfection  and  the  fine 
brownish  grey  which  is  without  banding,  and  has  only  the  folia- 
tion. They  blend  into  each  other,  but  the  light  grey  banded  is  on 
the  border  next  to  the  neighboring  rock  of  the  quarry  floor. 

As  we  study  this  rock  in  the  fragments  lying  about,  our  atten- 
tion will,  very  likely,  be  attracted  by  some  pieces  which  show 
what  at  first  appears  to  be  a  banding  directly  across  the  foliation 
of  the  rock.  There  are  alternating  bands  of  brownish  grey,  the 
normal  color  of  the  rock,  and  of  light  green.  On  examining  these 
greenish  bands  carefully,  we  see,  extending  through  each  and 
following  the  middle  plane,  a  vein  of  quartz;  from  this  vein  the 
green  extends  and  blends  into  the  brownish  grey.  From  this  ob- 
servation we  are  able  to  explain  this  peculiar  banding.  Where 
the  vein  now  is,  was  first  a  crack;  through  this  crack  water,  con- 
taining mineral  substances  in  solution,  slowly  percolated  and  soaked 
into  the  rock  on  either  side  of  the  crack,  carrying  in  mineral  matter 
or  withdrawing  mineral  matter  from  the  minerals  of  the  rock 
thus  changing  the  brownish  mica  and  producing  a  mineral  of  a 
light  green  color.  What  the  latter  is  cannot  be  made  out  exactly 
on  account  of  its  fineness.  It  is  probably  a  chlorite.  From  this 
we  see  how  percolating'  waters  may  produce  a  regular  and  distinct 
banding  within  a  massive  rock.  It  is  clear  from  this  that  banding 
does  not  always  indicate  a  former  sedimentary  state.  This  banding 
is  not,  however,  to  be  confused  with  the  finer  and  more  regular 
banding  in  this  same  rock  described  above.  Taken  as  a  whole  this 
rock  has  the  appearance  of  a  gneiss.  It  is  composed  of  quartz, 


84  GEOLOGY   OF   WORCESTER. 

feldspar  and  mica,  and  has  more  or  less  of  a  banded  structure 
especially  on  its  borders ;  nevertheless  from  other  considerations  we 
have  concluded  that  it  is  really  another  kind  of  rock,  as  will  appear 
in  our  discussion  of  the  band  marked  (12). 

(8)     Then  follows  a  narrow  band,  three  feet  wide, 

of  coarse  gneissoid  granite  similar  to  that  already 

described. 

(9)  Next  there  are  sixteen  feet  of  a  mixture  of  light  and  dark 
gneiss  or  schist  in  bands  one  to  two  feet  wide.  The  dark  bands 
are  of  a  biotite  or  black  mica  gneiss  or  schist;  the  light  bands  are 
partly  coarse  and  partly  fine,  with  dark  streaks  of  biotite  running 
through  them,  parallel  to  the  general  banding  of  the  quarry  rock. 
In  the  coarser  part  the  feldspar  masses  are  coated  with  light  yellow- 
ish mica  scales,  and  the  outside  rim  is  frequently  a  finely  granular 
zone.  These  two  facts  tell  us  of  a  crushing  and  crunching  of 
these  feldspars  by  which  mica  was  formed  out  of  the  feldspar  on 
the  friction  surfaces,  and  the  feldspars  were  granulated  more  or 
less  from  the  surface  towards  their  centres.  Little  facts  like  these 
will  frequently  reveal  to  us  the  dynamic  changes  to  which  rocks 
have  been  subjected. 

(10)  Continuing    our    observations     across     the 
containing        quarry,  we  next  find  a  band,  twenty-five  feet  wide, 
pebbie-iike       of  dark  grev  biotite  gneiss  or  schist  containing  lighter 

inclusion.  s     " 

colored,  narrow  bands,  a  few  inches  in  width.  These 
latter  show  a  marked  pebble-like  structure,  like  that  already  des- 
cribed. These  lighter  bands  are  also  frequently  crumpled  into 
many  fine  folds.  These  will  tell  us  somewhat  of  the  condition 
of  this  rock  when  this  folding  took  place.  For,  certainly,  these 
bands  could  not  now  be  so  folded  without  breaking. 

(11)  We  next  come  to  a  band  of  gneissoid  granite 
granite"1        about  seven  feet  in  width.     This  band  is  less  regular 

than  the  others,  while  the  rock  is  practically  the  same 
as  in  the  other  gneissoid  granite  bands.  This  constitutes  an  ir- 
regular lens-shaped  mass.  Into  it  extend  the  laminae  of  the  neigh- 
boring biotite  schist  in  delicate  streamers.  '  This  gneissoid  granite 
is  coarsely  crystalline,  the  feldspars  being  from  one  to  two  inches 
through,  and  the  foliation  is  traced  with  difficulty.  This  is  decided- 
ly granitic  in  structure,  appearance  and  position,  yet  possesses  the 
same  characteristics  found  in  the  gneissoid  granite  in  the  numerous 
bands  already  described. 


PSEUDO-METAMORPHIC    CONGLOMERATE.      GRANITE     INJECTED      INTO     MlCA 

SCHIST.    FROM  BALLARD'S  QUARRY,  NEAR  QUINSIGAMOND. 
NATURAL  SIZE. 


A  SLAB  FROM  QUINSIGAMOND  QUARRY,  SHOWING  RAGGED  ENDS  OF  THE 

SCHIST  LAMINAE  ON  EITHER  SIDE,  AND  INJECTED  COARSE  GRANITE, 

WITH   DIKE  OF  FINE  BROWNISH-GREY  GRANITE   CUTTING 

ACROSS  BOTH. 


GEOLOGY    OF    WORCESTER.  85 

(12)     Then  we  find  a  brownish  grey,  finely  grained, 
Finegrained,      biotite  rock,  more  or  less  clearly  foliated,  and  evi- 

brownish  grey 

granite.  dently  the  same  as  that  described  under  (7).  This 
band  is  but  three  to  four  feet  wide,  and  at  first  may 
seem  unimportant;  but  really  it  presents  facts  which  help  us  greatly. 
We  see  laminae  of  the  dark  banded  adjacent  gneiss  extending  into 
this  finer  grained  rock  and  growing  thinner  and  thinner  until  they 
end  in  sharp,  ragged  edges.  These  are  like  streamers  protruding 
into  this  neighboring  rock  so  gneissoid  in  appearance. 

In  fact  in  one  slab,  shown  in  the  illustration,  were  found  the 
opposite  ragged  ends  of  the  schist  where  the  schist  had  been  torn 
asunder,  and  this  fine  grained  rock  had  filled  the  break  and  gone 
into  the  small,  ragged  irregularities  without  disturbing  them  or 
breaking  the  sharp  edges.  Certainly  this  fine  grained  rock  must 
have  been  in  an  exceedingly  plastic  and  mobile  condition  to  thus 
fill  in  and  include,  without  breaking,  these  delicate  streamers  of 
the  schist.  Moreover  we  here  find  this  fine  grained  gneissoid  rock 
in  contact  with  the  coarse  gneissoid  granite.  Instead  of  constitut- 
ing a  band,  lying  alongside  of  the  granite  gneiss,  this  fine  grained 
rock  cuts  across  the  coarser,  as  if  the  latter  had  been  broken,  and 
the  finer  had  flowed  in  and  filled  the  break.  In  other  words, 
gneissoid  though  this  finely  grained,  brownish  grey  rock  may  be  in 
appearance,  its  occurrence  forces  us  to  believe  that  it  was  in  a 
molten  or  plastic  state  when  it  came  into  its  present  position.  It 
is,  then,  a  granite  and  not  a  gneiss. 

But  if  this  is  a  granite,  we  may  properly  inquire  why  it  is  that 
it  shows  such  perfect  banding,  as  is  described  under  (7),  especially 
next  to  its  contact  with  the  neighboring  rock.  As  was  pointed 
out,  this  banding  fades  out  as  the  distance  from  the  contact  in- 
creases, until  there  is  simply  an  indistinct  foliation.  The  border 
banding  is  the  result  of  selective  crystallization  during  the  cooling 
and  solidification  of  this  rock.  It  came  into  its  present  position 
while  in  a  molten  condition,  filling  fissures  or  cracks  in  the  neigh- 
boring rock.  The  latter  was  comparatively  cool,  though  probably 
still  heated  much  above  its  present  temperature.  The  molten  rock 
flowing  into  the  fissures,  was  cooled,  minerals  began  to  crystallize 
from  the  molten  rock,  not  all  together,  but  more  of  this  one  first, 
then  more  of  that  one.  Such  a  crystallization,  where  minerals  vary 
in  color,  must  produce  a  regular  variation  in  color,  and  hence  a 
banding.  But  while  we  may  see  that  the  banding  is  due  to  this 


86  GEOLOGY  OF  "WORCESTER. 

selective  crystallization,  it  is  not  so  easy  to  see  why  a  like  crystal- 
lization did  not  continue  through  the  mass.  There  is  hardly  more 
than  a  trace  of  it  in  the  midst  of  this  fine  grained  granite.  We 
conclude  then  that  the  cooler  neighboring  rocks  together  with  the 
motion  in  the  molten  mass  as  it  flowed  into  the  fissures,  produced 
conditions  specially  favorable  for  this  selective  crystallization,  con- 
ditions which  did  not  prevail,  or  prevailed  to  a  much  smaller  degree, 
during  the  crystallization  of  the  great  mass  of  this  granite  away 
from  the  surfaces  of  the  neighboring  rock. 

(13)  There  is  next  a  band  of  coarse,  white  gneis- 
Gneissoid        so^  granite ,  nine  feet  wide,  and  penetrated  bv  the 

granite.  '  . 

grey  granite  just  described. 

(14)  Then  three  feet  of  the  dark  biotite  schist 
followed  by  twelve  feet  of  narrow,  alternating  bands 
of  light  gneissoid  granite  and  dark  biotite  schist,  the 
granite  bands  fading  out  in  each  direction. 

(15)  Last  at  the  eastern  side  of  the  quarry  is  a 
granite.          band,  twenty-six  feet  wide,  consisting  almost  entire- 
ly of  the  gneissoid  granite  with,  now  and  then,  a 

narrow  band  of  the  biotite  schist,  this  gneissoid  granite  being 
essentially  the  same  as  the  gneissoid  granite  of  the  other  bands. 
We  have  thus  described,  band  by  band,  what  is  found  in  the  floor 
of  this  quarry  in  crossing  from  west  to  east  midway  between  the 
northern  and  southern  ends.  These  bands  make  up  together  a 
width  of  about  two  hundred  and  fifty  feet.  This  description  has 
been  given  in  such  detail  to  bring  out  clearly  the  exceeding  com- 
plexity here  presented,  and  to  give  a  clear  idea  of  the  various 
rocks  here  found. 

In   spite   of   the   many   apparently   disconnected, 
o/theTands       parallel  bands,  it  is  quite  evident  from  the  descrip- 
Timereia-        tions  that  all  of  these  rocks  may  be  classed  as  four; 
tKrocksthe       the  light  colored,  coarsely  foliated  gneissoid  granite, 
the  fine  grey  granite,   and  the  biotite  hornblende 
schist  or  gneiss,  and  the  coarse,  dark  grey  biotite  schist  with  pebble- 
like  inclosures.     From  the  descriptions  that  have  been  given,  it  is 
evident  that  these  four  rocks  bear  a  certain  time-relation  to  each 
other.     The  gneissoid  granite  includes  streamers  of  the  schist  and 
fills  into  little  irregularities  where  the  laminae  of  the  schist  were 
evidently  broken  or  torn  off;  then,  evidently,  the  schist  must  have 
been  there  first,  and  the  coarse  granite   must  have  been  in  an 


A  SLAB  OF  THE  FINE,  BBOWNISH-GREY  GRANITE,  SHOWING  PERFECT,  FINE 

BANDING  NEAR  THE   UPPER  EDGE,  AND   THE  FADING   OITT  OF  THE 

BANDING  AS  THE  DISTANCE  FROM  THE  CONTACT  INCREASES. 


GEOLOGY    OF   WORCESTER.  87 

exceedingly  plastic  state  to  have  thus  inclosed,  without  breaking, 
or  destroying,  these  fine,  delicate  streamers  of  the  schist.  We  must 
consequently  consider  the  coarse  gneissoid  granite,  in  spite  of  its 
marked  foliation  and  gneissoid  appearance,  a  true  granite.  It  is 
in  its  present  position,  younger  than  the  schist. 

But  we  have  already  pointed  out  that  the  finely  grained,  brown- 
ish grey  granite  not  only  includes  streamers  of  the  schist,  but  also 
cuts  across  the  coarse  gneissoid  granite,  evidently  filling  a  break 
in  the  latter.  The  brownish  grey  granite  is,  therefore,  younger 
than  either  of  the  other  two, — is  the  youngest  rock  met  in  the 
floor  of  this  quarry. 

Such  being  the  rocks  seen  at  this  locality  and  the  time  relation 
they  bear  to  each  other,  we  may  turn  to  the  oldest  one,  and  inquire 
what  may  have  been  the  history  of  these  bands  of  mica  schist. 
For  the  sake  of  simplicity,  it  will  be  well  for  us  to  divide  the  mica 
schist  here  found  into  three  kinds,  though  they  are  all  closely 
related,  and  consider  each  by  itself. 

There  is  first  the  mica  schist  consisting  of  alter- 

thlfmica  nating  bands  some  of  which  are  brownish,  and  some 
hornblende  are  light,  greenish  grey  in  color.  Although  the 
material  of  this  schist  is  thoroughly  recrystallized, 
its  banded  structure  and  composition  indicate  that  it  has  not 
always  been  as  it  now  is.  Its  composition  indicates  a  sorting  of 
rock  materials,  just  as  such  substances  are  sorted  by  currents  of 
water;  and  the  banded  structure,  though  now  appearing  in  crystal- 
line material,  also  points  back  to  rock  material  arranged  in  layers 
or  strata,  just  as  the  sands  deposited  by  water  are  so  arranged. 
Moreove'r,  the  material  deposited  by  water  in  one  layer  may  not 
be  entirely  of  one  mineral  because  of  imperfect  sorting.  One  layer 
may  be  sandy  and  at  the  same  time  argillaceous,  because  the  waters 
deposited  sand  and  clay  together;  another  layer  maybe  ferruginous 
because  iron  rust  was  deposited  with  the  sediments ;  in  like  manner 
another  may  be  calcareous.  If  layers  of  such  mixed  sediments 
undergo  a  recrystalli/ation  and  a  chemical  rearrangement,  as  the 
result  of  the  action  on  them  of  heated  waters  under  pressure,  one 
layer  may  differ  from  another  in  its  minerals,  and  hence  in  color 
and  appearance,  but  still  the  rock,  as  a  whole,  preserve  the  original 
bedding. 

This  in  brief  is  the  history  of  the  mica  hornblende  schist.  It 
was  once  sediment  deposited  in  layers  of  varying  composition  in 


88  GEOLOGY   OF  WORCESTER. 

the  shallow  waters  of  some  ancient  sea;  these  sediments  were 
covered  by  other  rock  strata,  also  deposited  in  that  ancient  sea, 
so  that  these  sediments  came  to  be  quite  deep  within  the  earth; 
there  they  were  recrystallized  through  the  agency  of  hot  waters 
under  pressure;  afterwards  they  were  brought  to  the  surface, 
where  now  they  appear,  by  the  removal  of  the  overlying  rocks. 
The  second  variety  in  the  schist  bands  is  in  part 
^^Snaa"*  a  scnist  closely  resembling  the  mica  schist  of  the 
pseudo-meta-  brownish  bands  of  the  hornblende  mica  schist.  The 
mica  is  a  dark  brown  biotite,  somewhat  coarser  than 
that  in  the  first  schist,  and  is  mixed  with  some  glassy 
quartz  and  probably  also  with  feldspar.  The  other  part  of  this 
schist,  and  that  which  distinguishes  it  from  the  other  schist,  is  a 
crystalline  mixture  of  quartz  and  feldspar  with  some  coarse  biotite 
mica.  This  granite-like  material  is  sometimes  distributed  in  quite 
well  defined  bands,  sometimes  narrow,  sometimes  wide,  parallel  to 
the  laminae  of  the  schist.  These  bands  are  nearly  white,  and  may 
be  seen  to  be  made  up  of  rounded  feldspars  with  coarse,  glassy, 
granular  quartz  between,  or  made  up  of  rounded  individuals  of  a 
mixture  of  the  same  pearly  white  feldspar  and  glassy  quartz  with 
black  biotite  wrapping  partially  around  these  individuals  and  more 
or  less  clearly  separating  them.  In  other  cases  these  bands  are 
less  regular  and  become  narrower,  and  more  or  less  discontinu- 
ous. The  rounded  feldspar,  or  feldspar-quartz  individuals,  then 
become  more  and  more  distinct  and  separated,  until  they  may 
be  found,  as  single  individuals  so  far  as  can  be  seen  on  a  cross- 
section,  inclosed  in  and  entirely  surrounded  by  the  schist.  Such 
individuals  are  generally  rounded  feldspars,  less  frequently  rounded 
masses  of  feldspar  and  glassy  quartz,  and,  least  frequently, 
rounded  individuals  of  massive,  glassy  quartz.  These  individuals 
vary  in  size  from  half  an  inch  or  less  up  to  two  or  three  inches  on 
their  largest  diameter,  and  they  are  uniformly  flattened  in  a  direc- 
tion parallel  to  the  lamination  of  the  schist.  The  appearance, 
which  we  have  tried  to  describe,  closely  resembles  what  might 
possibly  be  produced  by  the  recrystallization  or  metamorphism  of 
a  conglomerate,  the  schist  bands  and  mica  being  made  from  the 
fine,  sandy  feldspathic  material  separating  layers  of  pebbles,  and 
also  inclosing  individual  pebbles,  and  the  rounded  feldspar,  feld- 
spar-quartz, and  quartz  individuals  being  the  pebbles  flattened  by 
great  pressure.  It  is  a  subject  for  study  to  determine  whether 
or  no  this  part  of  the  quarry  rock  is  a  metamorphic  conglomerate. 


GEOLOGY   OF   WORCESTER.  89 

In  the  first  place  it  has  been  shown  that  there  is  much  granite 
in  this  quarry  rock  that  has  been  parallelly  injected  between 
the  laminae.  Some  of  the  continuous  bands  in  this  schist  are  not  to 
be  distinguished  from  these  true  granite  bands;  the  former  grade 
into  those  which  have  the  pebble-like  structure,  or  the  same  band 
may  be  partly  true  granite  and  partly  pebbly  in  structure.  More- 
over the  material  of  the  individual,  rounded,  pebble-like  masses  is 
identical  with  that  of  the  coarse  granite,  except  in  the  case  of  the 
massive,  glassy  quartz  particles;  and  this  resembles  matter  that  has 
crystallized  from  solution  or  aqueous  fusion  in  place,  rather  than 
material  that  has  been  worked  over  by  the  agents  of  air  and  water 
before  being  buried  in  the  rock.  After  careful  comparison  and 
study,  we  have  been  led  to  believe  that  all  of  these  light-colored 
bands  are  composed  of  granite,  and  that  the  pebbly  structure  is 
due  to  a  slow  crystallization  of  the  material  under  a  normal  pres- 
sure. Even  the  particles  which  are  entirely  separated  from  other 
granite  have  had  a  like  origin.  This  schist  is  not  really  different 
from  the  brown  mica  schist  with  the  hornblende  schist;  it  is  the 
brown  schist  into  the  midst  of  which  granite,  in  somewhat  thinner 
sheets,  has  been  injected,  producing  a  little  coarser  crystallization 
and  a  pebble-like  appearance. 

The  third  type  of  schist  found  in  this  quarry  is  a 

Chloritic  bio-  **  ,   J     , 

tite  schist  also     pure  biotite  schist  of  a  dark  grey,  almost  black  color, 
pseudo-con-       ^he  biotite  is  sometimes  coarse  and  sometimes  fine, 

and  produces  a  thinly  laminated  structure.  The  sur- 
faces of  these  laminae  are  frequently  greenish  in  color  indicating 
the  presence  of  chlorite,  so  that  this  schist  may  sometimes  be 
called  a  chlorite  biotite  schist.  Between  the  laminae  of  this  schist, 
also,  has  been  injected  granite,  sometimes  now  appearing  in 
regular  bands,  sometimes  in  discontinuous  sheets  and  separate,  dis- 
tinct lens-shaped  masses  an  inch  or  two  in  diameter.  This  injected 
granite  also  presents  the  pebble-like  structure.  This  schist,  like 
the  others,  is  but  a  recrystallized  sediment.  It  was  made  from 
layers  that  were  more  nearly  pure  clay,  and  contained  much  less 
sand,  hence  it  contains  little  or  no  glassy  granular  quartz. 

But  while  these  changes  were  taking  place  in  the 

Position  of  the     mineral  constituents  of  this  rock,  another  important 

bSevlde^ce     change  took  place.     In  going  across  the  quarry  we 

of  folding.       noted  that  every  thing  is  in  bands.     These  bands 

are  the  edges  of  the  laminae  or  layers  of  the  original 


90  GEOLOGY   OF   WORCESTER. 

rock,  and  of  the  intruded  granite  sheets.  These  layers  are  now 
nearly  vertical.  When  deposited,  the  layers  of  the  sediments  must 
have  been  horizontal,  or  nearly  so.  There  has  been  a  change  by 
which  these  layers  have  been  uplifted  from  a  horizontal  to  a  nearly 
upright  position.  The  meaning  of  this  is  that  these  beds  have  been 
folded.  To  make  this  clear  in  our  minds  let  us  think  of  strips  of 
paper,  four  feet  in  length  and  six  inches  or  so  in  width,  and  of 
different  colors;  then  let  the  red  ones  first  be  laid  down,  one  on 
top  of  another,  until  they  make  a  layer  one  half  inch  in  thickness; 
then  let  the  yellow  strips  be  laid  down  on  top  of  the  red  to  a  like 
thickness,  and  then  on  these  blue  strips,  and  on  these  still  others, 
until  the  sheets  of  paper  constitute  a  long,  narrow  pile  a  few 
inches  in  thickness.  That  the  paper  may  be  more  pliable,  let  it 
be  moistened;  then  think  of  the  ends  of  this  mass  of  paper  as 
pushed  towards  each  other;  the  paper  mass,  as  it  is  compressed, 
bends  into  several  folds;  and  if  the  ends  are  pushed  towards  each 
other  with  considerable  force,  the  sides  of  these  folds  will  be  pressed 
together  and  may  stand  vertically,  or  nearly  so.  If  now  with  a 
sharp  knife  we  cut  through,  horizontally,  midway  between  the  top 
and  the  bottom  of  these  folds,  and  remove  that  part  of  the  fold 
above  the  cut,  there  will  be  presented  to  us.  as  we  look  down  on  the 
folded  paper,  a  banded  appearance  where  each  band  is  really  the 
edge  of  a  layer  of  colored  paper,  and  these  layers  are  now  vertical, 
or  nearly  so,  because  of  the  folding  that  has  taken  place.  That 
is  exactly  the  meaning  of  the  nearly  vertical  layers  or  laminae  in 
the  schist  in  the  floor  of  this  quarry.  They  are  situated  in  the 
sides  of  more  or  less  extensive  folds,  the  upper  parts  of  which 
have  been  removed  by  the  great  erosion  to  which  this  area  has 
been  subjected  during  long  geologic  periods. 

In  going  longitudinally  through  the  quarry  we 
small  folds  in  frequently  observe  that  these  bands  are  wavy  in 
a  plane  at  right  form  jn  places  these  waves  are  so  abundant  that 

angles  to  that 

of  the  large       the  bands  present  the  appearance  of  having  been 

folds.  crumpled    into    many    folds,    many  of    them    even 

minute.     These   folds   are   not  to  be  confused   with 

those  larger  and  grander  ones  which,  if  built  up  as  they  formerly 

were,  would  extend  hundreds,  perhaps  thousands,  of  feet  into  the 

air.     These  smaller  folds  are  not  in  the  same  plane  with  the  larger 

ones;  and  while  the  larger  folds  indicate  a  compression  easterly  and 

westerly,  these  smaller  folds  indicate  a  compression  northerly  and 


A  BLOCK  OF  ROCK  AT  THE  BALLARD  QUARRY;    THE  WAVY,  FINE   BANDS 

ARE  SCHIST;  THE  COARSE,  LIGHT  BAND  IN  THE  MIDDLE  IS 

THE  COARSE   GNEISSOID  GRANITE. 


GEOLOGY   OF  WORCESTER.  91 

southerly.  Moreover  these  small  folds  are  on  such  a  scale  that 
we  may  see  a  succession  of  them.  So  nicely  was  this  minute 
folding  done  that  this  hard,  brittle  rock  material  was  bent  into 
innumerable,  closely  folded  waves  without  a  crack  or  fissure  ap- 
pearing throughout  the  mass.  This  minute  folding  may  be  seen 
to  better  advantage  on  the  ledges  in  the  woods  north  of  Gibbs 
street  and  just  east  of  the  gulch  at  the  foot  of  which  is  a  stone  bridge 
over  which  the  street  extends.  Such  folding  shows  us  very  clearly 
much  in  regard  to  the  condition  of  the  rock  when  this  folding 
took  place.  What  is  now  brittle,  unyielding  material  must  then 
have  been  exceedingly  pliable  and  flexible.  When  this  folding 
took  place,  these  rocks  were  deep  within  the  earth 

Condition 

of  the  rock       and  under  enormous  pressure ;   they  were  also  quite 
at  the  time       highly  heated  so  that  their  moisture  was  converted 

of  folding.  ° 

into  steam,  which  could  not  escape  on  account  of 
the  pressure.  Under  these  conditions  the  hard,  brittle,  solid  rock 
substance  became  as  yielding  as  clay  on  the  potter's  wheel — plia- 
ble, flexible,  bending  into  folds  innumerable  without  the  mixing 
or  mingling  of  band  with  band.  Sometimes  this  folding  took 
place  as  the  result  of  the  intrusion  of  the  granite,  for  the  schist 
laminae  were  forced  out  of  position  and  folded  around  the  granite 
mass.  More  frequently  granite  sheets  and  schist  laminae  were 
crumpled  together  into  a  series  of  beautiful  and  minute  folds. 
The  latter  foldirg  must  have  taken  place  after  the  granite  was  in 
place;  and  the  granite  offered  no  more  resistance  than  did  the 
schist. 

It  is  an  interesting  problem,  geologically,  to  de- 
Difflcuity         terrm'ne,   if  possible,   the   thickness   of   the   schists, 
Thickness  olf      anc^  fr°m  this  the  thickness  of  the  original  sedimen- 
the  schists.       taries  out  of  which  these  schists  were  formed.     With- 
in the  quarry  this  is  easily  done  by  simply  measur- 
ing band  after  band,  for  everything  is  there  uncovered.     Such 
direct  measurement  is  not  possible  when  we  deal  with  the  forma- 
tion as  a  whole,  because  by  far  the  larger  part  of  the  formation 
is  covered  by  sands,  gravels  and  till.     Neither  is  the  proportion 
of  granite  the  same  throughout  the  formation  as  it  is  in  the  quarry. 
In  fact  it  is  impossible  to  determine  with  any  degree  of  accuracy 
how  much  of  this  mixed  formation  is  either  schist  or  granite. 

Then  add  to  this  difficulty  that  which  comes  from  the  crumpling 
and  folding  in  this  formation  by  which  the  same  laminae  may  be 


92  GEOLOGY    OF   WORCESTER. 

repeated  again  and  again  without  our  being  able  to  identify  them 
as  the  same,  and  it  is  seen  that  the  problem  of  determining  the 
real  thickness  of  these  schists  is  well  nigh  insoluble. 

<luarry  and  t^e  Bolton  gneiss  formation  are 


Possible 

cause  of  the      an  excellent  illustration  of  an  idea  that  Prof.  Van 
folding  of        Hise  has  recently  expressed.     The  granite  constitut- 

the  schists. 

ing  more  than  half  of  the  rock  of  the  quarry  and  a 
large  part  of  the  whole  formation,  came,  as  we  have  said,  from  a 
greater  depth  within  the  earth  up  into  these  sedimentaries,  and 
there  solidified  and  remained.  By  this  transference  the  rock 
material  beneath  was  decreased,  and  that  nearer  the  surface  was 
increased.  The  outer  part  of  the  earth  became,  by  just  so  much, 
larger,  and  the  interior  became,  by  just  so  much,  smaller.  The 
sedimentaries  occupied  a  certain  space  in  the  earth;  to  have  forced 
into  their  midst  this  large  proportion  of  granite  must  have  neces- 
sitated a  giving  and  yielding  in  some  direction.  This  yielding  very 
likely  took  the  form  of  folding  and  elevation  of  the  beds  of  which 
there  is  so  much  evidence. 

We  have  alreadv  seen,  in  the  case  of  Millstone 

Connection  . 

between  the  Hill,  how  a  mass  of  molten  rock  may  rise  from  be- 
sheets  of  neath  through  some  fissure,  and  constitute  a  single, 
large  intrusive  mass  in  the  midst  of  sedimentaries, 
and  cool  and  crystallize  into  what  has  been  called  a  batholite. 
But  this  is  quite  different  from  what  is  presented  at  this  Quinsiga- 
mond  quarry.  Yet  the  two  occurrences  of  granite  are  not,  prob- 
ably, as  unlike  as  they  at  first  seem.  We  can  hardly  believe  that 
these  many  sheets  of  coarse  granite,  so  alike,  are  entirely  discon- 
nected. Yet  we  may  follow  a  schist  band  for  quite  a  distance, 
and  find  it  separating  two  granite  sheets  as  far  as  we  go,  or  as  far 
as  the  rock  surface  is  uncovered.  These  two  neighboring  granite 
sheets  have  no  visible  connection.  But  if  we  could  trace  these 
bands  down  into  the  earth,  we  should,  in  all  probability,  find  the 
schist  bands  growing  thinner  and  the  granite  sheets  thicker,  until 
the  former  ceased  to  separate  the  latter,  —  until  the  whole  rock 
became  granite.  There  we  should  find  an  extensive  mass  of  granite, 
constituting  a  large  batholite,  and  corresponding  to  the  single 
granite  mass  of  Millstone  Hill.  These  granite  sheets,  whose  edges 
we  see,  are  but  offshoots  from  this  larger  mass,  passing  up  between 
the  laminae  of  the  schist.  At  some  future  time,  as  the  rocks  now 
at  the  surface  in  this  region  are  removed,  lowering  the  surface 


GEOLOGY   OF   WORCESTER.  93 

more  and  more,  this  large  mass  of  buried  granite  may  become 

exposed. 

Having  traced  back,  step  by  step,  the  relation  of 
vtewoftha  the  rocks  in  this  quarry,  it  will  be  well  for  us,  in 
history  of  review,  to  state  the  facts  in  reverse  order  somewhat 

these  rocks.  . 

as  they  occurred. 

In  an  ancient  geologic  period  the  material  of  these  schists  was 
deposited  as  sediments  along  the  shores  of  the  continent  of  that 
time.  What  we  now  see  is  but  a  small  part  of  those  ancient  sedi- 
ments, because  of  the  great  erosion  to  which  they  have  been  sub- 
jected. These  sediments  were  covered  by  others  so  that  the 
former  were  deep  within  the  earth.  Then  followed  a  period  of 
disturbance  and  change.  The  strata,  originally  horizontal,  were 
folded  and  crumpled  into  both  large  and  small  folds;  and,  accom- 
panying the  folding,  perhaps  as  a  cause,  perhaps  as  a  result,  there 
rose  from  some  greater  depth  a  large  mass  of  molten  rock,  from 
which  offshoots  flowed  in  between  the  laminae  of  the  schist.  By 
the  folding  of  the  strata  and  the  intrusion  of  this  molten  granite, 
the  sediments  were  highly  heated  and  partially  dissolved  in  the 
hot  waters;  then  as  the  heated  rock  mixture  cooled,  the  partially 
dissolved  sediments  became  the  crystalline  schist  and  the  intrusive 
molten  rock  became  the  coarse,  light  grey  granite.  After  these 
rocks  had  cooled  considerably,  but  while  the  schist  was  still  flexible 
and  yielding,  again,  through  a  deep  fissure  in  the  rock  beneath, 
molten  rock  rose  into  the  midst  of  this  alternation  of  schist  and 
granite.  It  found  a  way  generally  through  openings  between  the 
laminae  of  the  schist,  or  between  schist  and  granite,  but  in  places 
flowed  into  breaks  across  both  laminae  of  the  schist  and  sheets  of 
the  granite.  This  later  intrusive  cooled  more  rapidly  than  did 
the  older  granite,  as  is  indicated  by  its  fine,  granular  texture.  This 
second  intrusive  is  the  fine,  brownish  grey  granite. 

Quarry  a  But  ^ne  rocks  in  this  quarry  are  not  the  only  objects 

mineral          of  interest.     On  a  small  scale  this  is  quite  a  mineral 

allty'         locality,  and  what  the  minerals  may  lack  in  quality 

they  make  up  in  number  of  species.     In  addition  to  the  constituent 

minerals  of  the  rock,  the  quartz,  the  mica    and  the  feldspar,  we 

have  pointed  out  that  in  the  coarse  granite  may  be  found  little 

zircon  crystals  of  a  brownish  color,  having  the  form 

of  a  four-sided  prism.     These  little  crystals  are  not 

limited  to  the  coarse  granite  in  any  special  part  of  the  quarry,  but 


94  GEOLOGY   OF   WORCESTER. 

may  be  found  in  any  part  of  that  rock,  if  only  one  looks  sharply 
enough,  and  uses  a  fairly  good  hand  magnifying  glass.  We  must 
class  this  zircon  as  an  original  mineral  in  this  rock;  and  it  evi- 
dently crystallized  before  the  mineral  matter  around  it  solidified, 
because  it  assumed  so  perfect  a  crystalline  form,  and  is  inclosed 
by  the  other  minerals. 

In  studying  the  coarse  granite  in  the  southern  part 
of  the  quarry,  our  attention  may  be  attracted  by  a 
peculiar  appearance  that  this  rock  sometimes  presents.  We  may 
see  on  its  surface,  here  and  there,  a  little  black  spot,  frequently 
not  more  than  one  sixteenth  of  an  inch  in  diameter.  The  rock 
around  this  black  spot  may  be  rusty  in  appearance,  which  may 
be  due  to  gummite  which  forms  from  the  alteration  of  allanite; 
but  more  peculiar  and  remarkable  is  the  manner  in  which  the 
rock  substance  is  puckered  around  the  black  spot.  This  appear- 
ance in  the  inclosing  rock  will  serve  to  distinguish  the  black  min- 
eral constituting  this  black  spot  from  black  mica.  After  examin- 
ing several  of  these  black  spots,  we  see  that  the  mineral  is  a  black 
mineral  having  a  coal-like  lustre,  and  is  in  the  form  of  prisms 
which  do  not  show  clearly  the  crystalline  form  of  this  mineral, 
but  belong  to  the  monoclinic  system.  The  prisms  here  vary  in 
length  from  the  merest  fraction  of  an  inch  to  half  an  inch,  and  in 
width  or  thickness  from  one  sixteenth  up  to  one  half  inch.  The 
quite  large  crystals  are  extremely  rare.  This  mineral  is  allanite, 
and  is  evidently  also  an  original  mineral,  inclosed  as  it  is  in  the 
very  midst  of  this  granite.  When  these  crystals  formed  and  the 
inclosing  rock  solidified,  for  some  reason  the  rock  puckered  around 
the  mineral  as  if  the  former  had  been  drawn  up  by  a  string. 

But  the  schist  bands,  as  well  as  the  granite,  are 

Hornblende          .  i  •       7         • 

and  amber  interesting  on  account  of  the  contained  minerals, 
colored  j^  nas  already  been  pointed  out  that  some  of  these 
bands  contain  both  light  green  and  dark  green  horn- 
blende. In  places  this  hornblende  is  coarse  enough  to  give  us  a 
clear  idea  of  the  appearance  of  this  mineral,  including  its  lustre 
and  cleavage.  In  these  hornblende  bands  are,  as  we  have  already 
pointed  out,  little,  dark  amber  colored  garnets  which  are  too  small 
to  be  seen  by  the  unaided  eye.  Even  under  the  magnifying  glass 
their  crystalline  shape  does  not  appear.  They  were  evidently 
formed  when  the  ancient  sediments  were  recrystallized  into  the 
schists. 


GEOLOGY   OF   WORCESTER.  95 

The  mica  schist  bands  are  also  of  interest  in  this 

ica  schist. 


Gametiferous     connectjon  an(j  wju  reward  us  for  careful  searching. 


Not  everywhere  in  these  bands  may  we  find  this 
next  mineral.  Only  now  and  then  does  it  appear  imbedded  in 
the  coarse,  dark  mica  schist.  This  is  a  beautiful  wine  colored 
garnet,  clear  as  crystal,  and  occurs  quite  abundantly  in  some  bands, 
and  in  good  sized  crystals.  We  long  to  get  them  out  of  the  rock, 
but  all  in  vain,  for  they  are  securely  held  by  the  inclosing  laminae 
of  the  schist.  If  we  try  to  break  them  out,  we  are  disappointed 
in  seeing  them  fly  to  pieces.  Nor  are  we  able  to  make  out  any 
exact  crystalline  form;  they  seem  to  be  slightly  flattened  in  the 
direction  parallel  to  the  laminae  of  the  schist,  thus  indicating 
the  pressure  to  which  this  rock  has  been  subjected.  They  were 
formed  in  the  laminae  of  the  schist  when  the  other  constituents 
were  crystallized,  and  are  original  minerals. 

But  this  somewhat  coarse  garnetiferous  schist  has 
more  to  rePav  us  f°r  our  study.  While  examining 
it  carefully  under  the  magnifying  glass,  our  attention 
is  attracted  to  little  scaly  particles  of  a  bright  metallic  lustre  and 
of  a  dark  grey  color.  These  particles  prove  to  be  very  soft,  and 
leave  a  mark  on  paper,  when  detached  from  the  rock  and  drawn 
across  the  white  surface.  These  are  evidently  little  scales  of  graph- 
ite. They,  like  the  garnets,  must  have  been  crystallized  in  the 
schist  along  with  the  other  minerals.  As  graphite  is  a  form  of 
carbon,  we  may  justly  ask  how  it  happens  to  be  here  in  this  schist. 
Carbon,  as  we  all  know,  is  the  foundation  of  plant  and  animal 
substances.  By  heat  the  carbon  contained  in  these  is,  to  a  greater 
or  less  degree,  set  free,  as  when  wood  is  converted  into  charcoal. 
If,  then,  in  these  ancient  sediments,  which  now  appear  in  the  form 
of  this  schist,  there  were  buried  leaves  and  twigs  and  other  vege- 
table or  animal  matter,  upon  being  heated,  as  they  must  have  been 
when  the  rock  was  recrystallized,  these  substances  must  have  been 
converted  into  free  carbon,  and  with  sufficient  heat  this  carbon 
must  have  crystallized  into  the  form  of  graphite.  We  shall  not 
think  wrongly,  then,  if  these  little  scales  of  graphite  stand  for  the 
ancient  plants  that  lived  while  the  material  of  this  rock  was  being 
deposited  in  an  ancient  geologic  time. 

Black  hom  ^n  ^n*s  same  dark,  coarse  mica  schist  we  notice, 

blende  in  here  and  there,  little  black,  shining  needles.  At  first 
they  make  us  think  of  tourmaline  prisms,  but  they 
do  not  show  the  ribbed  surface  characteristic  of 


96  GEOLOGY    OF   WORCESTER. 

tourmaline  and  show  a  distinct  cleavage,  and  so  we  recognize 
them  as  needles  of  black  hornblende.  It,  too,  is  an  original  mineral 
and  was  crystallized  with  the  rock. 

Another  specimen  from  this  quarry  is  worthy  of 
description,  not  because  of  any  new  mineral  in  it, 
but  because  of  the  pleasing  effect  of  the  combination 
of  minerals  within  it.  The  rock  contains  so  little  mica  and  so 
much  quartz  and  feldspar,  that  it  may  be  called  a  gneiss  instead 
of  a  schist.  It  is  thinly  laminated,  and  contains  biotite  with  fine 
muscovite.  The  quartz  and  feldspar  are  finely  granular,  and  so 
intimately  mix  as  to  be  indistinguishable.  This  granular  mixture 
constitutes  wavy  bands  separated  by  mica.  Through  the  granular 
mixture  are  distributed  many  wine  colored  garnets,  clear  as  crystal, 
giving  a  slight  rosy  tint  to  the  whole  surface  of  this  rock.  On 
only  one  occasion  have  we  found  this  phase  of  the  schist  at  this 
quarry,  and  then  not  in  large  quantity. 

The  minerals  thus  far  described  are  those  which 
original  and  must  have  been  formed  when  the  rocks  crystallized, 
minerals.  and  are  therefore  called  original.  There  are  other 
minerals  found  in  the  quarry  in  quite  different  occur- 
rences, indicating  that  they  have  been  formed  at  later  times. 
These  may  be  called  secondary  minerals. 

The  first  and  most  abundant  of  these  latter 
minerals  is  calcite.  It  is  found  coating  quite  large 
surfaces,  which  border  cracks  extending  through  the  ledge.  This 
mineral  is  found  especially  on  the  walls  of  the  eastern  side  of  the 
quarry.  It  is  generally  of  a  pure  white  color,  so  soft  as  to  be 
easily  scratched  by  the  knife  blade,  and  effervesces  freely  when 
touched  by  a  drop  of  hydrochloric  acid.  It  also  shows  a  perfect 
cleavage,  breaking  into  little  rhombohedra.  It  results  from  the 
decay  of  the  granite  and  schist.  Water  from  the  air,  containing 
carbonic  acid,  soaks  into  the  rock.  The  carbonic  acid  is  thus 
brought  in  contact  with  feldspar  and  other  minerals  containing 
calcium,  and  unites  with  the  calcium,  forming  carbonate  of  cal- 
cium, which  is  calcite.  The  calcite  is  then  carried,  in  solution  in 
the  water,  into  the  cracks,  and  there  is  deposited  on  the  surface 
as  a  coating,  at  times  even  filling  the  crack. 

Occurring  in  like  manner,  but  brought  in  and 
deposited  by  hot  water  containing  potash,  are  little 
quartz  crystals.  Some  of  these  are  colorless  and 


GEOLOGY    OF    WORCESTER.  97 

transparent,  clear  as  the  clearest  glass,  and,  though  small,  showing 
to  perfection  the  six-sided  prism  with  the  six-sided  pyramid.  Other 
of  the  quartz  crystals  are  dull  in  lustre,  either  milky  white  or 
slightly  rusty  in  color,  and  only  slightly  translucent.  These  varia- 
tions are  probably  duo  to  a  variation  in  the  conditions  which  pre- 
vailed during  the  formation  of  the  different  crystals. 

While  looking  for  the  calcite  or  the  quartz  crystals, 

Chabazite.  e 

we  may  notice  a  mineral  occurring  with  both  the 
calcite  and  quartz.  It  is  of  a  light  amber  color,  and  is  in  nicely 
defined  crystals.  These  crystals,  at  first  sight,  seem  to  be  cubical 
in  shape;  but  on  more  careful  examination  are  seen  to  have  angles 
not  exactly  right  angles.  These  little  amber  colored  rhombohe- 
dra  are  the  crystals  of  the  mineral  chabazite.  Occurring  in  the 
fissures,  it  is,  evidently,  a  secondary  mineral,  and  has  been  formed 
from  other  minerals  of  the  rock,  and  crystallized  from  the  perco- 
lating waters.  Nice  crystals  of  this  mineral  may  be  justly  prized 
as  coming  from  this  locality.  They  constitute  beautiful  little 
specimens. 

Closely  related  to  the  last  is  another  mineral  also 

Stilbite 

found  in  the  fissures  in  the  eastern  part  of  the  quarry. 
It  belongs  to  the  same  family  of  minerals,  and,  like  the  chabazite, 
is  the  result  of  the  chemical  decomposition  of  certain  minerals  in 
the  granite  or  schist.  It  is  not,  however,  so  frequently  found, 
and  its  crystals  are  not  so  noticeable,  and  hence  do  not  so  quickly 
attract  the  attention.  This  mineral  is  almost  pure  white  in  color, 
and  in  fine,  thin,  flat  blade-like  crystals,  pointing  in  almost  every 
direction  and  crossing  each  other  at  various  angles,  forming  a  mat 
or  network.  These  crystals  are  also  marked  by  a  pearly  lustre 
from  which  it  received  its  name.  It  is  the  mineral  stilbite.  It 
has  thus  far  been  found  only  as  a  thin  coating  on  fissure  surfaces. 
But  if  we  examine  carefully  the  crevices  and  fis- 
sures in  the  eastern  part  of  the  quarry,  we  may  find 
one  which  has  been  entirely  filled  with  mineral  matter.  This  mineral 
filling  may  be  clearly  distinguished  from  the  inclosing  rock.  This 
mineral  constitutes  a  vein.  It  is  glassy  in  lustre,  and  white  or 
light  green  in  color.  It  is  so  hard  that  the  knife  blade  barely 
scratches  it.  We  see,  from  the  shining  surfaces  that  are  presented 
as  the  specimen  is  moved  in  the  light,  that  the  vein  is  made  up 
of  fine  crystals.  After  breaking  out  some  of  one  of  these  veins, 
we  may  be  fortunate  enough  to  find  a  place  where  the  mineral 


98  GEOLOGY   OF   WORCESTER. 

did  not  completely  fill  the  fissure,  and  there  the  distinct  crystals 
appear.  When  free  from  iron  rust,  they  are  white  or  of  a  light 
green  color.  The  little  crystals  are  little  plates,  tabular  in  shape, 
standing  on  their  edges  at  various  angles  and  without  uniformity 
in  direction.  They  make  a  confused  mass  in  which,  however,  we 
are  able  to  distinguish  the  shape  of  the  individual  plates  under 
the  magnifying  glass.  We  see  clearly  three  and  four  sides  on  the 
edges  of  these  little  plates,  and  can  fill  in  the  remaining  sides  from 
the  regularity  of  form,  thus  making  out  that  these  plates  have  six 
sides,  each,  around  the  edge.  This  mineral  is  known  as  prehnite. 
It  has  been  formed  by  chemical  decomposition  in  the  neighboring 
granite,  and  has  been  deposited  in  these  cracks  and  fissures  through 
the  agency  of  water. 

After  visiting  this  quarry,  more  or  less  frequently, 

Pyrrhotite.  .  .  •   «  i  •  •         «  •    i 

during  eighteen  years  one  might  reasonably  think 
that  all  the  minerals  occurring  here  had  been  observed;  neverthe- 
less on  visiting  this  quarry  on  the  14th  of  April,  1900,  another, 
not  before  observed  here,  was  found  in  considerable  quantity. 
The  rock  in  the  vicinity  of  it  was  marked  by  extreme  rustiness 
and  by  a  honey-comb  appearance.  This  is  due  to  the  fact  that 
the  mineral  rusts  rapidly,  thus  coating  the  neighboring  rocks,  and 
is  quickly  removed  from  the  rock,  leaving  the  spaces  it  filled  as 
cavities.  These  changes  require  oxygen  from  the  air,  and  so  do 
not  take  place  until  the  mineral  is  within  reach  of  the  air.  On 
examining  this  mineral  it  is  seen  to  occur  in  a  quartz  vein  within 
a  schist  band.  The  mineral  is  of  a  bronzy  color;  has,  when  free 
from  rust,  a  bright  metallic  lustre ;  is  quite  heavy,  having  a  specific 
gravity  of  4.6;  and  is  slightly  attracted  by  the  magnet.  These 
properties  indicate  clearly  that  the  mineral  is  pyrrhotite  or  mag- 
netic pyrites.  As  this  mineral  frequently  contains  nickel,  and  is, 
in  fact,  the  principal  ore  of  nickel,  the  question  immediately  arose 
whether  or  not  this  particular  pyrrhotite  contains  nickel.  In  it 
may  be  found  nickel  enough  to  give  a  decided  test.  As  this  min- 
eral occurs  inclosed  in  the  glassy  quartz  of  a  quartz  vein,  it  must 
be  as  old  in  its  position  as  is  the  quartz  and  must  have  got  into 
its  position  much  as  the  quartz  did.  The  quartz  vein,  like  other 
veins,  is  the  filling  of  a  crack  or  fissure;  the  quartz  of  it  was  brought 
in  and  there  deposited  through  the  agency  of  water.  In  like 
manner  must  this  mineral  have  been  deposited  along  with  the 
quartz. 


GEOLOGY    OF   WORCESTER.  99 

On  visiting  this  quarry  we  must  not  expect  that 
these  minerals  will  be  so  obvious  that  we  cannot 
overlook  them;  we  must  have  our  sharp  eyes  on,  use  these  to  the 
best  advantage,  go  slowly  in  our  search,  and  examine  everything 
through  the  hand  magnifying  glass.  Working  in  this  way.  and 
being  constantly  on  the  lookout  for  any  fact,  we  notice  that,  now 
and  then,  the  surface  or  edge  of  a  slab,  which  has  been  broken  out 
of  the  quarry,  has  a  greenish  yellow  color.  The  first  thought  is 
that  a  fine  moss  is  growing  on  this  surface;  but  through  the  mag- 
nifying glass  a  single  glance  is  sufficient  to  remove  this  idea.  The 
surface  is  seen  to  be  covered  by  a  thin  coating  of  glassy  crystals, 
shining  and  flashing  in  the  sunlight.  Some  of  the  crystals  are 
large  enough  to  be  seen  as  transparent,  greenish  yellow  prisms. 
There  is  no  mistaking  this  mineral,  it  is  epidote.  It  is  also  a  second- 
ary mineral  formed  from  other  minerals,  and  deposited  as  a  coating 
on  these  surfaces.  We  may  in  other  localities  find  larger  crystals 
of  epidote,  but  we  shall  not  find  any  that  are  prettier. 

Another  mineral  we  may  find  in  the  eastern  part 

Vermiculite.  rf 

of  the  quarry,  also  coating  surfaces,  is  almost  black 
in  color  on  the  outside,  but  within  is  dark  green.  It  occurs  in 
small  tufted  masses,  crowded  together  over  the  surface.  On  ex- 
amination under  the  glass  we  see  that  each  tuft  is  a  mass  of  little 
black  or  dark  green  plates  pressed  together.  Where  one  of  these 
tufts  happens  to  be  broken,  it  is  seen  to  be  a  mass  of  scales  resem- 
bling mica  scales,  and  we  may  think  that  possibly  this  is  only  a 
variety  of  mica.  On  heating  a  few  of  these  tufts  in  the  blowpipe 
flame,  this  mineral  begins  to  squirm  and  twist  and  turn  inside  out, 
as  if  it  were  truly  animated  and  were  suffering  excruciating  pain. 
Not  so,  however;  this  strange  effect  is  but  the  passing  off  of  water 
that  was  held  confined  in  the  substance.  Because  of  this  twisting 
and  squirming  when  heated,  the  mineral  was  called  vermiculite, 
a  wormlike  stone.  The  neighboring  town  of  Millbury  has  long 
been  noted  among  mineralogists  as  affording  this  mineral. 

Quite  closelv  related  to  the  last  mineral,  and  like 

Prochlorite.  J 

it  occurring  as  a  thin  coating,  is  still  another  mineral. 
It,  too,  is  of  a  dark  green  color,  and  so  soft  as  to  be  easily  cut  by 
the  finger  nail,  when  the  green  color  more  clearly  appears.  It  is, 
at  its  best,  a  mat  of  fine,  dark  green  scales,  and  is  readily  recognized 
as  prochlorite.  This  mineral  is  frequently  found  as  a  secondary 
mineral  in  connection  with  hornblende  rocks,  and  it  is  possible 


100  GEOLOGY   OF   WORCESTER. 

that   here    it   has   been    derived    from    the    hornblende    in    the 
schist. 

There  are  other  minerals  that  have  been    found 

corpperPpyiui,  here>  but  onlv  in  sma11  quantity.  There  is  iron 
malachite,  pyrites,  almost  omnipresent  in  the  rocks  of  Worces- 
actinoute  ^cr,  easily  recognized  by  its  light  brassy  color  and 
its  hardness.  Then  there  is  chalcopyrite,  or  copper 
pyrites,  occurring  more  abundantly  on  the  surfaces  of  joints  and 
seams,  and  distinguished  from  iron  pyrites  by  its  darker  yellow, 
brassy  color  and  by  its  softness,  being  easily  scratched  by  the 
knife  blade.  Associated  with  the  last  mineral  may  be  seen,  at 
times,  in  small  quantity,  the  green  and  blue  carbonates  of  copper, 
malachite  and  azurite.  The  copper  in  them  was  derived  from  the 
copper  pyrites,  and  carbonic  acid  was  carried  in  by  the  percolat- 
ing water.  Last  of  all  there  is  a  beautiful  form  of  actinolite  that 
has  been  found  here.  Some  years  ago  the  quarrymen  opened  up 
quite  a  large  vein  of  smoky  quartz  which  contained  radiating 
masses,  an  inch  or  a  little  more  in  diameter,  of  a  light  green  min- 
eral. These  masses  consisted  of  fine,  silky  fibres  radiating  from  a 
centre,  and  the  mineral  was  as  beautiful  and  delicate  as  the  finest 
silk.  These  radiating  masses  were  exposed  as  the  workmen  broke 
into  this  vein.  The  mineral  was  recognized  as  a  very  fine  actino- 
lite. Either  because  work  was  given  up  in  that  part  of  the  quarry 
or  because  the  supply  was  exhausted,  that  form  of  actinolite  has 
not  since  been  found.  There  is  another  form  of  actinolite 
consisting  of  long,  black  needles,  more  or  less  radiating,  which  is 
still  found  in  different  parts  of  the  quarry. 

In  this  somewhat  extended  studv  of  the  rocks  and 

This  quarry 

a  hand  sped-  minerals  at  the  Quinsigamond  quarry  it  must  be 
men  of  a  large  borne  in  mind  that  it  has  not  been  a  study  simply 
of  that  small  detached  area,  though  that  would 
abundantly  repay  us,  but  a  study  of  a  large  area  extending  many 
miles  to  the  south,  and  several  miles  to  the  east  beyond  the  bounds 
of  Worcester.  We  have  been  studying  a  somewhat  large  hand 
specimen  of  the  Bolt  on  gneiss  instead  of  the  whole  broad  area. 
In  doing  this  we  have  not  observed  some  facts  and  minerals 
appearing  in  other  parts  of  this  formation  which  do  not  appear 
here.  To  complete  our  study  we  must  notice  these. 

Going  out  on  the  Boston  and  Albany  railroad  to 
^ne  eas*  from  the  deep  cut  at  Bloomingdale,  we  come 
to  a  wooden  bridge,  painted  red,  spanning  the  tracks. 


GEOLOGY   OF   WORCESTER.  101 

This  bridge  rests  on  ledge  at  either  end.  The  rock  of  this  ledge 
is  a  compact,  massive  mica  schist,  and  a  part  of  the  great  schist 
and  granite  area  which  is  the  subject  of  our  study.  On  breaking 
into  the  ledge  here,  in  certain  bands,  we  find  the  schist  containing 
quite  long,  glassy  needles,  colorless  or  rusty,  at  times  so  abundant 
and  crossing  each  other  in  so  many  directions  as  to  make  a  per- 
fect network.  The  needles  are  but  a  small  fraction  of  an  inch  in 
thickness,  but  in  length  may  be  a  full  inch  or  more.  On  examin- 
ing these  needles  through  the  magnifying  glass  they  are  seen  to 
consist  of  exceedingly  fine,  glassy,  colorless  fibres  extending  from 
end  to  end  of  the  needles.  A  nice  specimen  of  this  rock  with  the 
contained  mineral  will  add  to  any  mineral  collection,  for  it  shows 
a  phase  of  this  mineral  not  frequently  met  in  this  vicinity.  This 
mineral  has  been  called,  on  account  of  its  structure,  fibrolite. 
It  also  bears  the  name  Sillimanite,  in  honor  of  Benjamin  Sil- 
liman,  the  first  professor  of  geology  at  Yale. 

There  is  another  locality  of  considerable  inter- 
est  within  the  Bolton  gneiss.  Leaving  the  electric 
car  at  Bramanville,  we  go  to  that  part  of  Millbury 
called  Old  Common,  thence  follow  the  road  to  the  south.  Im- 
mediately, at  the  first  small  rise,  we  notice  the  rusty  color  of 
the  dirt  in  the  road;  and  then,  at  the  top  of  this  rise,  we  see  in 
the  road  and  by  the  side  a  rock  of  peculiar  appearance  compared 
with  those  before  met  in  this  study.  Moreover,  in  the  bank  on 
the  right  we  may  see  more  of  this  rock,  here  marked  by  its  ex- 
tremely rusty  appearance,  and  by  the  ease  and  rapidity  with 
which  it  evidently  decays.  Still  farther  on,  also  on  the  right, 
there  is  an  opening  made  into  the  bank  where  the  rock  is  better 
exposed,  and  more  easy  of  access.  There  are  also  many  frag- 
ments which  have  been  thrown  out,  and  these  may  help  us  greatly 
in  our  study. 

As  before,  we  notice  the  extremely  rusty  appearance  of  the  whole 
ledge,  indicating  the  presence  of  some  iron  mineral  easily  oxi- 
dized by  the  air.  The  surface  of  the  ledge  and  of  the  fragments 
in  the  piles  on  either  side  is  very  rough  and  ragged,  yet  presents 
a  certain  regularity  or  banding  in  this  raggedness.  On  examina- 
tion we  see  that  this  is  due  to  the  uneven  weathering  or  decay  of 
the  rock,  some  layers  weathering  away  more  rapidly,  and  thus 
leaving  the  others  projecting  irregularly  and  producing  this  uneven, 
rusty,  ragged  surface. 


102  GEOLOGY   OF  WORCESTER. 

Let  us  now  break  into  the  rock  to  obtain  a  fresh  surface  for 
examination  and  study.  The  rock  has  a  medium  fine,  granular 
texture,  and  presents  many  small,  glistening,  smooth,  white  surfaces 
telling  us  of  the  crystalline  grains  making  up  this  rock.  Some- 
thing in  the  glistening  of  these  grains  makes  us  suspicious  that 
these  are  not  like  those  ordinarily  seen  in  the  rocks  of  this  vicin- 
ity. We  test  them  with  the  knife  blade,  and  find  them  to  be  quite 
soft.  We  next  put  a  drop  of  hydrochloric  acid  on  the  rock,  and 
we  are  told  by  the  effervescing,  which  is  the  escape  of  carbonic 
acid  gas,  that  this  rock  is  limestone;  and  by  the  many  shining 
cleavage  surfaces,  that  it  is  a  crystalline  limestone.  Knowing  this 
we  now  understand  wrhy  this  rock  weathers  so  rapidly  and  so 
irregularly.  The  limestone  is  soluble  in  water,  especially  in  water 
containing  carbonic  acid,  and  so  the  mineral  substance  of  the 
rock  is  constantly  carried  away  in  solution  by  the  waters  coming  in 
contact  with  it.  But  if  we  notice  carefully  we  see  that  not  all  of  the 
particles  effervesce  when  touched  by  the  acid.  These  latter  parti- 
cles are  different  in  composition,  and  do  not  pass  into  solution  in  the 
water;  hence  where  they  are,  the  weathering  of  the  rock  surface  is 
not  so  rapid.  The  projecting  bands  on  the  weathered  surfaces  are 
then  due  to  these  other  minerals  which  are  not  so  easily  removed. 

As  we  break  into  this  rock  in  different  parts  of  the  ledge,  we 
find  that,  however  rusty  it  may  be  on  the  outside,  it  is  free  from 
rust  at  a  short  distance  from  the  surface.  We  also  find  that  there 
is  considerable  variation  in  the  color  of  the  fresh,  unrusted  rock; 
but  taken  as  a  whole  it  is  of  a  grey  color.  But  while  noticing 
the  color  of  the  rock  as  a  whole,  we  observe  a  variety  of  colors 
in  the  different  grains.  Let  us  carefully  examine  these  under  the 
magnifying  glass. 

There  is  first  the  mineral  of  a  light  brassy  color 
the'tamatone  anc^  metallic  lustre,  occurring  in  fine  grains  and  dis- 
tributed in  every  part  of  the  rock.  We  instantly 
recognize  it  as  iron  pyrites.  Its  abundance,  together  with  the  ease 
and  rapidity  with  which  the  limestone  about  it  is  removed,  thus 
bringing  it  to  the  surface,  ex'plains  the  extremely  rusty  appearance 
of  the  rock  and  of  the  neighboring  road. 

We  next  notice  some  dark  grey  particles,  at  times 

limestone11       a  sixteenth  of  an  inch  or  more  in  diameter.     They 

shine  with  a  metallic  lustre,  and  are  found  to  be 

very  soft  when  touched  with  the  knife  blade.     They  are  scratched 


GEOLOGY   OF   WORCESTER.  103 

even  by  the  finger  nail.  These  particles  are  in  the  form  of 
fine  scales,  and  may  be  poked  out  by  means  of  a  pin.  One  of 
these  scales,  rubbed  across  the  page  of  the  note-book,  leaves  a 
mark  like  that  of  the  graphite  in  the  pencil.  These  are  scales  of 
graphite,  and  retain  their  lustre  even  in  the  rusty  surfaces,  be- 
cause the  agents  of  the  air  and  water  have  no  chemical  effect  on 
them.  They  point  back  to  organic  matter  that  was  contained  in 
this  limestone  before  it  was  crystallized.  In  the  crystallization 
of  the  minerals  of  the  rock,  the  carbon  contained  in  these  substances 
was  crystallized  into  graphite. 

But  while  looking  at  the  graphite  particles,  we 
notice,  because  of  its  abundance,  a  light  green  min- 
eral. It  is  sometimes  granular  and  sometimes  in  the 
form  of  light  green,  glassy  prisms  or  blades,  which  are  best  seen 
on  weathered  surfaces,  and  present  more  or  less  completely  the 
outline  of  definite  crystals.  This  is  light  green  actinolite  of  the 
hornblende  family. 

other  minerals  ^n  addition  to  these  minerals,  there  may  be  found 
in  the  lime-  in  this  limestone  a  grey  feldspar  in  rounded,  cleav- 
able  particles,  an  half  inch  or  so  in  diameter,  com- 
pletely surrounded  by  the  limestone  particles;  and  also  a  white 
feldspar  in  fine  particles  which  remain  after  the  carbonate  of  cal- 
cium has  been  dissolved  by  acid.  There  are,  also,  a  reddish  brown 
pyroxene,  not  easily  recognized  under  the  hand-glass,  and  small 
garnets  and  magnetite.  From  what  has  been  said  it  is  evident 
that  this  limestone  is  far  from  pure,  but  the  rock  is  all  the  more 
interesting  to  us  on  that  account. 

But  we  may  reasonably  ask  in  regard  to  the  extent 
°^  ^s  limestone,  for  we  are  not  accustomed  to  think 
of  limestone  as  occurring  in  this  vicinity.  As  a  re- 
sult of  the  examination  of  the  neighboring  ledges,  we  find  that 
this  limestone  constitutes  a  narrow  band,  twenty  feet  or  so  in 
width,  striking  thirty-seven  degrees,  east  of  north  and  dipping 
forty  degrees  west.  This  band  may  be  traced  to  the  southwest 
from  where  we  first  find  it  a  short  distance,  and  then  we  lose 
all  trace  of  it.  The  limestone  is  only  a  local  rock.  On  examining 
the  rock  in  the  ledges  east  and  west,  in  which  the  limestone 
must  be  imbedded,  we  find  it  to  be  very  like  to  the  Bolton 
gneiss  with  its  injected  granite.  In  fact  by  studying  the  ledges 
of  the  intervening  area,  we  may  trace  a  direct  connection 


104  GEOLOGY   OF  WORCESTER. 

between  these  ledges  in  Millbury  and  those  at  the  quarry  near 
Quinsigamond.  The  ledges  in  these  two  somewhat  distant  local- 
ities belong  to  the  same  formation;  and  the  rock-floor  under  the 
intervening  territory  consists  of  the  Bolton  gneiss.  This  lime- 
stone is  evidently  but  a  narrow  band  or  thin  layer  in  the  Bolton 
gneiss,  and  was  crystallized,  with  the  formation  of  the  included 
minerals,  when  that  formation  was  crystallized  out  of  sediments, 
other  lime  **ut  *^e  Millbury  limestone  is  not  the  only  one 

stone  areas  m     in  the  Bolton  gneiss.     There  is  another  one  in  Web- 
the  Bolton       s^er      j^  js  situated  a  mile  or  so  west  of  the  great 

lake.  In  Northboro  is  another,  near  Ghost  Hill. 
At  this  locality  may  be  seen  the  remains  of  the  old  lime-kiln  in 
which  the  limestone  was  once  converted  into  lime.  In  Bolton  is 
still  another.  This  has  been  noted  for  a  long  time  as  a  rich  min- 
eral locality.  Near  the  centre  of  Boxboro,  still  farther  to  the 
north,  is  another.  Here  also  is  the  old  lime-kiln.  Here  are  large 
masses  of  beautiful  scapolite  lying  in  the  dump,  thrown  out  as  so 
much  waste  material. 

As  we  have  said,  all  of  these  limestone  areas  are 
°]faeatoue.e      within  the  region  underlaid  by  the  Bolton  gneiss,  and 

the  limestone  in  each  case  is  but  local  in  extent.  The 
limestone  area  is  but  a  few  hundred  feet,  at  the  most,  in  diameter. 
These  limestone  masses  within  the  Bolton  gneiss  may  represent 
deposits  from  calcareous  springs.  They  form,  however,  a  series  of 
beds  extending  clear  across  the  state  and  may  have  been  more 
continuous  formerly  than  now,  and  have  been  formed  by  the 
accumulation  of  organic  remains.  These  deposits  were  afterwards 
recrystallized  and  the  included  minerals  formed,  when  they  and 
the  surrounding  rock  were  more  or  less  deep  beneath  the  earth's 
surface. 


CHAPTER  V. 
SHREWSBURY  DIKE. 

While  this  is  intended  as  a  study  of  the  rocks  of  Worcester, 
nevertheless,  we  have  been  led,  more  or  less  frequently,  beyond 
the  limits  of  the  city,  because  of  the  extension  of  these  rocks,  and 
because  geological  phenomena  are  not  limited  by  town  boundaries. 
There  is  a  small  region  just  beyond  the  border  of  Worcester,  in- 
cluded within  the  area  of  the  Bolton  gneiss,  which  will  well  repay 
our  study.  To  this  we  next  invite  your  attention. 

Taking  the  electric  car  for  Shrewsbury,  and  having  reached  the 
eastern  side  of  Lake  Quinsigamond,  we  pass  the  cemetery  on  the 
right,  and  then,  somewhat  beyond,  come  to  a  road  on  the  left.  At 
this  corner,  we  pass  into  the  field  at  the  northwest,  and  immediately 
find  two  quite  different  rocks.  One  is  a  rusty,  thinly 
laminated  mica  schist  containing  the  mineral  fibrol- 
ite  in  small  masses  of  shining,  glassy  needles.  The 
other  rock  of  this  ledge  is  of  a  dark  grey,  almost  black,  color; 
is  of  a  finely  granular  texture,  massive  in  structure,  and  is  cut 
irregularly  by  many  joints  into  quite  small  angular 
H°8rcnilietnde  blocks-  Within,  this  rock  has,  in  spite  of  its  out- 
wardly massive  appearance,  somewhat  of  a  schistose 
structure  due  to  the  parallel  arrangement  of  the  mineral  particles, 
producing  a  tendency  to  split  in  the  direction  of  the  arrange- 
ment. As  we  handle  these  pieces  of  rock,  they  seem  to  us  heavier 
than  similar  pieces  of  average  rock,  and  this  makes  known  to  us 
its  somewhat  high  specific  gravity.  Under  the  magnifying  glass 
we  see  that  this  rock  is  largely  composed  of  one  mineral,  black 
in  color  and  distributed  in  fine,  blade-like  crystals  lying  parallelly 
to  each  other,  and  thus  producing  the  schist  structure  already 
noticed.  This  black  mineral  we  recognize  from  its  lustre  and 
structure  as  black  hornblende.  Distributed  among  the  particles 
of  hornblende  we  see  other  particles,  white  in  color  and  glassy  in 
lustre.  On  weathered  surfaces  these  white  particles  become  dull 
white  from  decay,  showing  that  they  are  feldspar;  but  they  are 


106 


GEOLOGY    OF    WORCESTER. 


so  fine  that  we  cannot  determine,  by  simply  looking  through  the 
hand-glass,  whether  they  are  of  orthoclase  or  of  plagioclase  feld- 
spar. This  rock,  because  of  its  schist  structure,  and  because  of 
the  abundance  of  hornblende  in  it,  may  properly  be  called  a  horn- 
blende schist. 

But  in  addition  to  this  study  of  the  rock  itself,  we  must  study 
its  relation  to  the  neighboring  rusty  mica  schist  found  in  the  same 
ledge.  We  first  determine  the  extent  of  the  hornblende  schist — 
that  it  is  but  a  narrow  mass,  six  to  eight  feet  in  width,  with  the 
rusty  schist  on  either  side  of  it,  and  wrapped  ov^r  by  the  rusty 
schist  above.  The  accompanying  illustration  shows  the  face  of  the 
ledge  as  it  appears  above  the  ground. 


The  hornblende  schist  varies  in  its  structure,  being  generally 
very  massive,  in  spite  of  the  parallelism  in  the  arrangement  of  the 
hornblende;  but  next  to  the  rusty  mica  schist  this  massiveness 
gives  place  to  a  truly  schist  structure,  the  rock  becoming  even 
fissile,  and  the  folia  are  parallel  to  the  laminae  of  the  mica  schist. 
If  one  does  not  look  carefully,  he  may  think  that  one  rock  blends 
into  the  other;  but  the  presence  or  absence  of  the  hornblende  serves 
to  clearly  indicate  where  one  rock  ends  and  the  other  begins. 
We  here  carefully  study  these  two  rocks,  where  we  may  observe 
them  together  and  on  such  a  small  scale,  that  we  may  clearly  see 
the  relation  of  one  to  the  other. 

Seeing  another  outcrop  to  the  northwest,  and  only 
seventy-five  to  one  hundred  feet  away,  we  go  to 
that.  We  find  the  same  two  kinds  of  rock,  the 
massive,  somewhat  finely  grained  hornblende  rock,  cut  into  angu- 
lar blocks  by  joints  and  fissures,  and  the  thinly  laminated  rusty 
mica  schist.  But  let  us  look  for  the  relation  which  they  bear  to 
each  other  in  position,  as  we  did  in  the  study  at  the  other  outcrop. 


Second  out- 
crop. 


DlOBITE  FROM  SHREWSBURY   DlKE.      ORIGINAL,  5  INCHES  BY  4. 


GEOLOGY    OF   WORCESTER.  107 

The  western  side  of  this  outcrop  is  nearly  vertical,  and  twelve  to 
fifteen  feet  high;  this  affords  us  an  excellent  opportunity  to  de- 
termine the  relative  positions  of  these  rocks.  Starting  at  the  base 
of  the  ledge,  at  the  southern  end,  we  find  the  finely  grained,  streaked, 
massive  hornblende  schist  rising  above  the  ground,  three  or  four 
feet  only,  and  sloping  down  to  the  north  into  the  ground  out  of  sight. 
Above  this  and  resting  on  it,  we  find  the  rusty  mica  schist,  six 
to  eight  feet  thick,  also  sloping  down  to  the  north,  parallel  to  the 
hornblende  schist,  and  disappearing  beneath  the  ground.  Next 
above  this  mica  schist,  and  also  sloping  down  like  the  others,  is 
another  bed  of  hornblende  schist  of  greater  thickness  than  the 
others,  and  the  last  that  we  can  see  on  the  western  side  of  the 
outcrop.  On  going  up  on  this  outcrop,  and  thence  to  the  north, 
at  a  distance  of  a  few  feet,  we  come  to  more  outcrops,  all  of  rusty 
schist.  If  then  we  put  the  facts  here  presented  into  a  drawing, 
they  will  appear  in  this  way. 


From  this  we  see  that  the  relative  positions  of  these  two  rocks 
in  this  outcrop  are  quite  different  from  what  they  were  in  the 
first  outcrop  but  a  few  rods  distant.  Of  the  identity  of  the  two 
rocks  in  one  outcrop  with  those  in  the  other  there  can  be  no  doubt. 
Before  trying  to  comprehend  the  full  meaning  of  these  facts,  let 
us  search  for  additional  material  to  assist  us  in  the  study. 

Looking  to  the  north  we  see,  at  but  a  short  dis- 
Third  outcrop,     tance  and  under  two  large  oak  trees,  a  large  outcrop. 

In  this  we  find  a  massive  hornblende  rock.  At  first 
sight  it  appears  quite  different,  because  of  its  coarseness,  from 
that  hornblende  rock  found  in  the  other  two  outcrops.  It  is  of 

a  dark  grey  color,  blotched  with  white,  and  is  cut  by 

Description  .  °     J 

of  this  phase  of     many  joints  or  fissures  into  irregular,  angular  blocks. 

thehrocklende     But  little  of  the  schist  structure  appears  in  it,  though 

there  is  somewhat  of  parallelism  in  the  arrangement 

of  the  coarse  hornblende.     On  the  weathered  surfaces  the  horn- 


108  GEOLOGY    OF   WORCESTER. 

blende  frequently  projects,  showing  that  it  is  less  rapidly  removed 
by  the  air  and  water  than  is  the  material  of  the  white  blotches. 
These  blotches  are  dull  white  in  color,  frequently  oval  in  shape, 
an  inch  or  more  in  diameter,  and  arranged  so  that  the  longer  axis 
is  parallel  to  the  direction  of  the  hornblende  masses.  We  break 
into  the  rock  to  obtain  a  fresh  surface  for  examination.  The 
blotches  are  much  less  conspicuous  here  than  on  the  weathered 
surfaces.  The  mineral  constituting  these  blotches  is  in  fine  grains, 
which  now  and  then  show  the  cleavage  surfaces  of  feldspar.  They 
are,  then,  granulated  crystals  of  feldspar,  and  probably  indicate  a 
crushing  or  shearing  in  this  rock  mass.  Within  some  of  these 
granulated  feldspars  may  be  seen  very  fine,  olive  green  needles 
pointing  approximately  in  the  direction  of  the  longer  axis  of  the 
feldspar.  These  are  needles  of  actinolite.  All  these  facts  indicate 
an  arrangement  of  these  minerals  under  pressure.  We  also  exam- 
ine the  cleavage  surfaces  of  the  feldspar  particles  for  striations, 
for  these  will  prove  that  the  feldspar  is  plagioclase  and  not  ortho- 
clase.  We  derive  very  little  satisfaction  from  this  search  because 
the  bright  shining  surfaces  are  so  rarely  seen.  After  much  search- 
ing, we  conclude  that  the  cleavage  surfaces  do  not  show  the  stria- 
tions,  and  so  we  are  left  in  doubt  whether  this  feldspar  ma  be 
orthoclase  or  plagioclase  without  the  twinning  which  produces  the 
striations;  but  from  the  basic  composition  of  the  rock  we  may  ex- 
pect the  feldspar  to  be  plagioclase.  That  this  is  so  is  proved  by 
the  ease  with  which  it  fuses  and  by  its  effect  on  polarized  light. 
There  are  patches,  also,  somewhat  abundant  in  places,  of  a  brown 
colored  mica.  The  hornblende  and  the  triclinic  feldspar  are  the 
essential  minerals  of  this  rock.  It  is  then  a  diorite.  When  it  con- 
tains the  brown  mica,  it  is  a  biotite  diorite.  The  finer  hornblende 
rock  in  the  first  two  outcrops  is  simply  this  same  rock  having  a 
finer  texture. 

Thus  far  we  have  noticed  only  the  essential  min- 

^h™d°ortten      erals  °f  this  rock;    but  during  our  study  we  have 

very  likely  noticed  a  bronzy  colored  mineral  in  fine 

particles  distributed  through  the  mass  of  the  rock  in  places.     This 

is  pyrrhotite,  a  sulphide  of  iron.     With  this  may  be  seen  a  yellow, 

brassy  colored  mineral  of  metallic  lustre  also  in  small  quantity 

and  in  grains.     This  is  chalcopyrite  or  copper  pyrites. 

Then  there  is  also  the  common  iron  pyrites.     But 
much  more  abundant  than  these,  and  occurring  in 


SHREWSBURY  DIKE.  LEDGE  OF  INDURATED  TALC  BY  SIDE  OF  ROAD. 


GEOLOGY    OF    WORCESTER.  109 

quite  a  different  manner,  is  another  mineral.     It  is  found  coating 

the  surfaces  of  the  rock,  filling,  or  partially  filling,  certain  joints  by 

which  the  rock  is  cut.     The  mineral   is  white,  and 

Scapoliteasa 

secondary  occurs  in  long,  bladed  crystals  crossing  each  other 
mineral  in  the  so  as  {o  form  a  network  on  the  surface.  This  is  the 

diorite. 

mineral  wernerite  or  scapohte,  and  quite  pretty 
specimens  have  been  found  by  breaking  away  the  rock  along 
some  of  these  joints. 

Still  another  mineral  found  here,  but  in  smaller 

^hf  diortte11      particles,  is  magnetite.     The  presence  of  this  is  made 

known  by  powdering    some  of    the  rock,  and   then 

putting  the  ends  of    the  magnet  in  the    powdered  rock.       The 

magnetite  adheres  to  the  ends  of  the  magnet. 

But  here,  as  in  the  other  outcrop,  let  us  note  the  relation  which 
the  hornblende  rock  bears  to  the  neighboring  rock.  The  latter  is 
the  same  rusty  mica  schist  containing  fibrolite,  which  was  found 
in  the  other  two  outcrops;  and  here  the  rusty  schist  is  found  east 
and  west  of  the  hornblende  rock.  From  this  we  are  able  to  derive 
an  idea  of  the  width  of  the  hornblende  rock.  It  is  from  fifty  to 
one  hundred  feet  wide  in  different  parts  of  this  somewhat  long 
outcrop. 

But  again,  before  trying  to  understand  further  in 

Fourth  out-  J 

crop  of  the       regard  to  this  rock,  let  us  determine  whether  we  have 

h0rrocknde       found  a11  that  there  is  of  it-      TakinS  the  direction 

in  which  it  is  most  likely  to  be  extended,  we  follow 

the  adjacent  road  to  the  north,  a  few  hundred  yards,  and  come 

to  another  outcrop.     Here  the  ledge  is  in  the  form  of  a  knoll. 

Next  to  the  road  appears  the  bare,  vertical  surface  of  the  ledge, 

about  ten  feet  high,  presenting  an  excellent  opportunity  for  study. 

Going  around  this  knoll,  on  the  southwestern  and  western  sides, 

we  find,  as  before,  the  thinly  laminated,  rusty,  fibrolitic  mica  schist 

leaning  against  the  rock  of  the  knoll.    The  latter  rock  is  massive  and 

without  structure,  of  a  dark  grey  or  rusty  grey  color, 

Description  of  .  .    .  . 

the  rock  of  the     and   is   cut   by   many    joints   into   angular   blocks. 

fourth  out-       iis  surface  is  more  or  less  uneven,  because  of  the 

unequal  weathering.     On  attempting  to  break  the 

rock,  we  find  it  not  brittle,  but  tough  and  soft,  yielding  beneath 

the  hammer.     The  fresh  surface  of  the  rock  is  of  a  dark  grey  color, 

of    finely  granular  texture,  and  with  almost  no  foliated  or  schist 

structure.     As  we  handle  the  rock  we  notice  a  smooth,  soapy  feel, 


110  GEOLOGY   OF  WORCESTER. 

and  find  that  the  rock  is  easily  and  deeply  scratched  by  the  knife 
blade.  Under  the  magnifying  glass  we  see  the  great  mass  of  the 
rock  to  be  of  a  dark  grey  colored  substance  in  which  we  may  de- 
tect, now  and  then,  a  bright  blade  of  hornblende,  like  that  seen 
in  the  rock  of  the  last  two  outcrops.  Most  of  this  dark  grey  sub- 
stance is  dull,  or  made  up  of  glistening  particles  too  fine  to  be 
distinguished  under  the  glass.  There  are  also  small  particles  of  a 
light  green  and  olive  green  color,  glassy  in  lustre,  and  soft  enough 
to  be  easily  scratched  by  a  pin.  Particles  of  magnetite  may,  now 
and  then,  be  seen,  of  a  dark  grey  color  and  bright  metallic  lustre. 
These  may  be  separated  from  the  rock  substance  by  powdering 
the  rock,  and  then  stirring  the  powder  with  the  ends  of  the  magnet. 
In  this  way  we  find  the  magnetite  quite  abundant,  while  the 
coarse  hornblende  rock  under  the  oak  tree  contains  little  of  this 
mineral. 

While  the  description  just  given  applies  to  the  great  mass  of 
the  rock  of  this  knoll,  on  breaking  into  it  at  different  places  we 
find  considerable  variation.     At  times  the  rock  has  somewhat  of 
a  schist  structure,  especially  in  the  northern  part  of    the  ledge 
bordering  the  road.     This  variety  has  the  same  soapy  feel,  dark 
grey  color,  and  softness.     Under  the  glass  we  see  in  the  midst  of 
the  dark  grey,  here  and  there,  bright  blades  of  hornblende,  and 
the  same  light  green  mineral  as  before,  but  the  schist  surface  has 
an  irregular  or  knotted  appearance,  as  if  something  were  buried 
oiivine  in        Jus*  Deneatn  the  surface.     After  considerable  break- 
fourth  out-       ing  of  the  rock  we  may  succeed  in  finding  that  these 
little  knots  are  produced  by  an  inclosed  particle  of 
glassy  lustre  and  amber  color.     This  mineral  proves  to  be  oiivine. 
There  is  still  another  mineral  here,  but  it  cannot  be  seen  even  under 
Dolomite  in       *ne  magnifymg  glass.     Its  presence  is  made  known 
fourth  out-       by  dropping  a  piece  of  the  rock  into  hydrochloric 
crop'  acid.     There   may   then   be   a   slight   effervescence, 

which  becomes  quite  rapid  on  heating  the  acid.  This  is  a  charac- 
teristic of  the  mineral  dolomite,  showing  that  it  is  distributed  in 
fine  particles  through  this  rock. 

We  find  here,  also,  a  part  of  the  rock,  especially  near  the  surface, 

of  a  light  grey  color,  sometimes  of  a  greenish  grey,  bordered  on 

the  weathered  surface  by  a  yellowish  layer  a  quarter  of  an  inch 

or  more  in  thickness.     Under  the  magnifying  glass 

Tremolite.  ,  ,.    ,  ,     .  ,  , 

the  massive,  light  grey  rock  is  seen  to  be  made  up, 


GEOLOGY   OF   WORCESTER.  Ill 

in  considerable  part,  of  a  finely  fibrous  mineral,  the  fibres  in  small 
converging  masses,  of  a  faint  olive  green,  grey,  or  white  color. 
This  mineral  is  a  form  of  tremolite.  On  the  outside,  where  this 
mineral  comes  in  contact  with  moisture  and  agents  of  air,  the 

rock  becomes  yellowish  in  color,  soft  and  soapy  to 
Fibrous  talc.  the  touch,  though  still  remaining  fibrous.  These  are 

characteristics  of  the  mineral  talc.  Quite  pretty, 
though  small,  specimens  of  fibrous  talc  may  here  be  obtained. 

We  will  now  consider  the  rock  of  the  knoll  as  a 

fourth  out-       whole.     The  light  green,  soft  mineral,  found  in  the 

crop  a  massive     different  specimens,   is  light  green  talc;    the  dark 

grey,  massive  part  is  also  largely  talc,  containing 
blades  of  hornblende.  The  rock  as  a  whole  is  then  a  massive  talc. 
But  it  is  of  further  interest  because  it  presents  to  us  several  tran- 
sitions. The  meaning  of  the  association  of  the  hornblende  and 
talc  is  that  the  former  is  in  the  process  of  change  into  the  latter; 
the  little  particles  of  olivine,  formerly  distributed  through  the  whole 
rock  as  we  have  found  them  in  one  specimen,  have  changed  into 
light  green  talc;  while  carbonic  acid  from  the  air,  together  with 
magnesium  and  calcium  from  the  hornblende  or  olivine,  have 
formed  dolomite;  in  part  also  the  hornblende  near  the  surface  has 
changed  to  tremolite,  and  then  the  latter  has  further  changed  into 
a  fibrous  talc.  From  these  statements  it  is  evident  that  the  rock 
is  now  quite  different  from  what  it  originally  was,  the  hornblende 
and  olivine  being  the  only  minerals  which  we  can  consider  as  be- 
longing to  the  original  rock.  The  former  serves  to  connect  this 
ledge  with  the  outcrops  already  studied  a  few  hundred  yards  to 
the  south. 

But,  again,  we  search  in  the  neighboring  fields  for 
Fifth  outcrop,     more  ledges  that  may  be  connected  with  the  line 

thus  far  traced.  Going  easterly  across  one  field  and 
into  the  next,  under  a  large  oak  tree,  a  little  to  the  south,  we  find 
ledges.  In  outward  appearance  the  rock  is  very  dark  grey,  in 
places  almost  black.  The  surface  is  uneven  from  unequal  weather- 
ing, and  presents  a  dull  white  mineral  in  irregular  streaks,  separ- 
ating the  small  masses  of  a  black  mineral,  the  latter  standing  out 
on  the  surface.  We  break  into  the  rock,  and  find  the  same  coarse, 
black  hornblende  in  bladed  masses,  with  some  hornblende  of  a 
light  green  color,  and  finely  granular,  glassy  feldspar  showing,  now 
and  then,  a  cleavage  surface  under  the  glass.  Clearly  this  ledge 


112  GEOLOGY   OF  WORCESTER. 

belongs   with   those  already   studied   on   the  other   side    of    the 
road. 

Thence  we  go  to  another  oak  tree  somewhat  north- 
sixth  outcrop,  easterly,  in  the  same  field,  and  but  a  short  distance, 
and  there  more  of  the  hornblende  rock  appears;  and 
then  a  little  farther  on,  still  more;  then  crossing  a  small  brook 
and  going  towards  the  Shrewsbury  road,  still  more;  thence  going 
on  the  Shrewsbury  road  and  starting  up  the  hill,  perhaps  one  fourth 
the  distance  up,  we  come  to  an  excellent  cutting  in  the  ledge  made 
when  the  electric  road  was  built.  Here  is  a  fresh  surface  ready  for 
our  study. 

We  note  first  the  marked  schist  structure,  on  ac- 
count  of  which  the  rock  has  a  smooth  surface  paral- 
lei  to  the  road,  which  is  here  about  north  and  south. 
Tms  surface  of  the  rock  is  almost  vertical,  dipping 
eighty  degrees  to  the  west.  It  also  presents  a  marked 
streaked  appearance,  the  streaks  being  so  regular  and  long  as  to 
give  one  the  impression  of  alternating  white  and  black  lines.  These 
alternating  lines  pitch  down  to  the  north  at  an  angle  of  fifty-five 
degrees  from  the  horizontal.  The  black  streaks  prove,  on  exami- 
nation, to  be  black  hornblende,  and  the  white  show  the  finely 
granular,  glassy  feldspar  with,  now  and  then,  an  ungranulated 
particle  in  the  midst.  Again  evidently  we  have  found  more  of 
the  coarse  hornblende  rock  which,  by  outside  pressure  and  shear- 
ing, has  been  given  this  schistose  structure.  On  examining  more 
of  this  ledge  but  a  few  feet  from  the  road,  we  find  that  much  of 
it  is  lacking  in  the  schist  structure,  and  in  part,  at  least,  is  much 
more  feldspathic.  In  spite  of  this  there  is  no  difficulty  in  consider- 
ing it  as  belonging  with  the  line  of  outcrops  already  found.  We 
may  here  notice  also  a  few  narrow  bands,  of  much  lighter  color, 
running  through  the  midst  of  the  hornblende  rock.  These  are  the 
edges  of  granite  dikes.  The  granite  is  later  than  the  hornblende 
rock,  and  simply  fills  fissures  made  in  this  rock  at  some  time.  We 
may  also  find  on  the  surface  of  joints  white  blade- 
like  crystals  of  wernerite  or  scapolite,  exactly  like 
those  in  the  ledge  under  the  first  oak  trees  near  the  beginning 
of  this  line  of  outcrops. 

Another  fact  to  be  noticed  here  is  the  presence  of 
*^e  same  rusty  schist  a  short  distance  beyond,  in 
the  bank  of  the  road.  The  meaning  of  this  is  that 


GEOLOGY    OF   WORCESTER.  113 

this  hornblende  rock  has  no  great  width  here;  but  the  width  is 
not  so  easily  made  out  here  as  at  the  first  outcrops,  because  of 
the  loose  earth  covering  the  surrounding  surfaces  of  the  rock. 

But  again  we  continue  our  search,  going  up  the 
roac^  towards  Shrewsbury,  to  the  first  road  branch- 
ing to  the  left;   turning  into  the  field  at  the  right, 
we  go  to  the  top   of  the  hill   near  some  large  trees,  and  there 
find  outcrops.     At  first,  just  before  reaching  the  top  of  the  slope, 
we  find  the  rusty  mica  schist,  and  then,  but  a  few 
feet  easterly>  a  dark  Srey  massive  rock,  cut  by  many 
joints,  in  various  directions,  into  irregular,  sharply 
angular  blocks.      In  spite  of  the  massiveness  we  see  a  streaked- 
ness,   even  on  weathered  surfaces,   telling  of  a  schist  structure 
within  due  to  the  parallel  arrangement  of  the  min- 
of  the  rock       erals.     Breaking  into   it,  we   find   the   rock   closely 
of  the  eighth      resembling  the  hornblende  rock  at  the  first  two  out- 
crops in  this  line.     This,  too,  we  may  call  a  horn- 
blende schist,  being  made  up  of  the  black  hornblende  and  finely 
granular  feldspar,  which  are  arranged  in  parallel  and  irregularly 
alternating  folia.     These  outcrops  we  put  into  the  line  of  outcrops 
we  are  following.     The  limited  extent  of  this  rock  to  the  west  is 
indicated  by  the  occurrence  of  the  rusty  mica  schist  but  a  few  -feet 
distant;  probably  the  extent  to  the  east  is  no  greater.     The  horn- 
blende rock  is  at  the  most  but  a  few  hundred  feet  wide. 

Leaving  this  locality  and  returning  to  the  road 
where  we  left,  we  pass  on  to  Mr.  Moen's  estate, 
taking  a  direction  ten  degrees  west  of  north;  at  a 
distance  of  a  quarter  to  a  third  of  a  mile  we  come  to  a  ledge  the 
rock  of  which  is  familiar  in  appearance.  But  on  the  way  we  must 
notice  the  rock  seen  in  the  ledges  of  the  fields  through  which  we 
pass.  The  ledges  are  quite  numerous,  and  all  are  of  the  rusty, 
mica  schist  with  which  we  have  become  so  well  acquainted. 

The  rock  of  the  special  ledge  to  which  we  have  come  is  of  a  dark 
grey  color,  of  mottled  or  blotched  appearance.  The  blotches  are  of 
a  dull,  dirty  white  color,  an  inch  or  so  along  the  longer  diameter, 
and  are  roughly  oval  in  shape,  on  a  background  of  very  dark  grey. 
The  surface  of  the  ledge  is  more  or  less  uneven,  at  times  pitted, 
because  of  the  unequal  weathering.  Breaking  into  the  rock  for 
a  fresh  surface,  we  see  that  the  dark  grey  or  black  mineral  is  simply 
hornblende,  in  finely  bladed  masses,  and  the  white  is  granular 
9 


114  GEOLOGY   OF   WORCESTER. 

feldspar.     In  fact  this  rock  is  identical  with  that  seen  in  the  third 
outcrop  near  the  beginning  of  the  line. 

If  we  wish  to  follow  farther,  by  going  fifteen  de- 
outcrop.         &rees  west  of  nortn  an(*  an  eighth  of  a  mile,  we  may 
find   still  another  outcrop  of   the  hornblende  schist 
type,  identical  with  those  already  described. 

It  is  evident  from  these  descriptions  that  we  have 
these  ten  out-  been  dealing  with  rocks  closely  related  though  in 
crops  to  each  separated  outcrops.  We  notice  also  that  these  out- 
crops are  narrow,  and  are  bordered  on  either  side  by 
the  rusty  mica  schist,  which  is  found  in  a  considerable  area  in  the 
surrounding  country;  that  these  outcrops  do  not  follow  a  line 
parallel  with  the  strike  of  the  laminae  of  the  schist,  but  are  in  a 
zigzag,  crossing  this  strike  several  times.  It  is  evident,  then,  that 
this  hornblende  rock  in  these  different  outcrops  does  not  constitute 
a  bed  or  layer  in  the  rusty  schist,  for  then  it  would  follow  the 
strike.  We  must,  then,  consider  this  hornblende  rock  an  eruptive 
which  is  also  indicated  by  its  composition  and  also  by  the  facts 
observed  in  the  first  two  outcrops.  In  the  form  of  a  molten  rock 
it  came  into  its  present  position.  It  is  possible  that  all  these  out- 
crops constitute  a  single  dike,  and  that  the  dike  appears  in  these 
separated  outcrops  because  of  the  loose  earth  which  covers  and  hides 
the  intervening  parts  of  the  dike.  By  this  we  mean  that  there  was 
one  fissure  following  the  line  we  have  traced,  and  into  this  the 
molten  rock  came,  filling  it,  and  there  solidified.  It  is  more  prob- 
able, however,  that  these  outcrops  are  offshoots  from  a  considerable 
mass  of  this  rock  beneath,  and  that  there  is  no  surface  connection 
between  most  of  these  outcrops.  The  meaning  of  this  is  that  at 
some  ancient  geologic  time,  there  rose  from  some  greater  depth  in 
the  earth  a  considerable  mass  of  molten  basic  rock  into  the  midst  of 
this  rusty  mica  schist,  when  the  latter  was  far  beneath  the  surface 
of  the  earth;  from  this  larger  mass  of  molten  rock  offshoots  forced 
their  way  between  various  laminae  of  the  schist;  in  due  time  the 
whole  mass  and  offshoots  cooled  slowly  and  crystallized.  After  the 
crystallization,  in  the  course  of  events,  the  schist  and  the  inclosed 
eruptive  rock  were  subjected  to  a  common  pressure  or  shearing, 
crushing  the  feldspars  of  the  latter  to  granulated  masses,  and  giving 
to  it  a  marked  schist  structure  parallel  with  that  of  the  neighboring 
mica  schist.  Still  later  the  joints  were  produced,  cutting  across  the 
schist  structure,  possibly  by  the  cooling  down  from  the  heated 


GEOLOGY   OF   WORCESTER.  115 

condition  produced  by  the  shearing  and  crushing.  Then  as  time 
went  on,  the  surface  of  the  earth  above  was  gradually  lowered  by 
weathering  and  by  the  removal  of  the  rock  material  by  running 
waters,  until  the  earth's  surface  reached  the  schist  into  which  these 
offshoots  of  molten  rock  had  penetrated,  thus  exposing  them,  sepa- 
rated by  areas  of  rusty  mica  schist,  but  not  exposing  the  larger 
mass  of  which  they  are  offshoots,  because  the  surface  has,  as  yet, 
not  gone  quite  low  enough. 


CHAPTER  VI. 

ROCKS  IN  THE  BALLARD  FIELD  AT  QUINSIGAMOND 

AND 
RELATION  OF  BOLTON  GNEISS  TO  THE  CARBONIFEROUS  QUARTZITE. 

Having  now  described  and  studied  the  rocks  of 
the  Ballard  quarry,  as  representing  the  Bolton  gneiss, 
together  with  the  rocks  of  some  areas  of  interest,  also  belonging 
to  this  formation,  but  not  represented  among  the  rocks  of  the 
quarry,  we  are  led  to  seek  for  the  relation  that  this  Bolton  gneiss 
bears  to  the  Carboniferous  rocks,  which  have  already  been  studied, 
and  which  lie  just  west  of  it.  If  we  can  determine  this  relation,  we 
may  then  assign  this  gneiss  to  its  proper  place  in  the  historical 
sequence  of  the  rocks  of  the  earth. 

In  seeking  to  determine  this  relation,  let  us  go  di- 
rectly west  from  the  Ballard  quarry,  through  the  fields 
north  of  Gibbs  street.  We  find  there  many  outcrops,  and,  in  places, 
see  alternating  schist  and  granite  bands  beautifully  and  marvel- 
lously crumpled,  showing  that  they  have  been  subjected  to  a  com- 
mon north  and  south  compression.  In  this  study  everything  is 
perfectly  clear  as  far  west  as  the  gulch,  at  the  foot  of  which  is  a 
stone  arch,  over  which  is  Gibbs  street.  Crossing  this  gulch,  it  is 
evident  that  we  have  reached  a  critical  place,  where  we  must  care- 
fully and  closely  study,  if  we  would  aright  interpret  the  facts  here 
presented.  The  critical  area  extends  from  this  gulch  west  to 
Providence  street,  and  north  from  Gibbs  street  across  one  field 
into  the  next,  as  far  as  any  outcrop  appears. 

It  will  be  necessary  for  us,  first,  to  describe  the  rocks  found  in 

this   area,  as  they  of    themselves  are  very  interesting.     In  this 

description  we  begin  with  the  rocks  on  the  east  side  of  Providence 

street,. nearly  opposite  Doane  street.     The   rock  here  has  recently 

Granite  of        been  cut  into  to  furnish  building  stone,  thus  afford- 

the  Baiiard       ing  an  excellent  opportunity  for  its  study.  This  rock 

is  of_a  dark  grey  color,  due  to  the  abundance  of 


GEOLOGY   OF  WORCESTER.  117 

very  dark  brown  biotite  mica.  This  mineral  is  distributed  in  thin, 
long,  narrow,  blade-like  masses,  giving  to  the  rock  a  streaked  ap- 
pearance. These  mica  masses  are  also  parallel  to  each  other,  and 
so  abundant  that  they  give  to  the  rock  a  marked  foliation,  par- 
allel to  the  lamination  in  the  neighboring  schists.  Because  of  this 
foliation,  the  rock  easily  splits  into  well-defined  slabs.  The  folia- 
tion is  in  a  plane  extending  nearly  north  and  south,  and  nearly  verti- 
cally. The  foliation  has  a  dip  of  eighty-five  degrees  to  the  west. 
As  we  walk  over  the  surface  of  the  ledge,  we  are  stepping  on  the 
edges  of  these  folia.  The  other  minerals  of  the  rock  are  quartz  and 
feldspar,  so  finely  granular  that  they  are  not  to  be  distinguished 
from  each  other  by  the  unaided  eye.  Under  the  magnifying 
glass  we  may  see  particles  of  quartz  somewhat  larger  than  the 
average  and  slightly  tinted.  The  whole  appearance  is  of  a  rock 
that  has  been  severely  crushed  and  sheared,  granulating  the  quartz 
and  feldspar,  and  arranging  the  mica  in  blades,  giving  to  the  rock 
a  foliated,  almost  schistose,  structure.  That  this  rock  is  a  granite 
is  indicated  by  the  position  it  occupies  relative  to  the  neigh- 
boring rocks,  and  by  its  position.  That  it  is  younger  than  the 
quartzite,  found  in  the  field  nearby,  is  made  evident  by  its  con- 
taining inclusions  of  that  quartzite  in  the  most  northerly  outcrop, 
a  little  north  of  the  eastern  end  of  Upsala  street.  An  inclusion 
of  the  Carboniferous  mica  schist  is  also  found  in  this  granite;  the 
latter  is  then  younger  than  the  schist.  The  eastern  boundary  of 
this  granite  is  quite  accurately  indicated,  because  the  outcrops 
are  numerous;  but  the  western  border  is  uncertain,  because  the 
rock-floor  is  almost  completely  covered  by  till  from  Providence 
street  to  Vernon  street,  where  the  Carboniferous  phyllite  ap- 
pears. On  the  south,  the  boundary  of  this  granite  is  concealed 
by  the  sands  of  the  Blackstone,  and,  on  the  north,  by  a  thick 
covering  of  till.  While  we  cannot  make  out  accurately  the  bounds 
of  this  granite,  we  are  certain  that  it  has  no  great  extension.  It 
is  simply  a  local  rock,  but  has  a  character  of  its  own.  It  is  a 
distinct  variety  of  granite  found  within  the  borders  of  Worcester. 
On  the  surface  of  the  ledge  opposite  Doane  street 
hfthe* granite  are  seen  ^gn^  colored,  narrow  bands,  that  contrast 
with  the  darker  granite.  The  rock  of  these  is  nearly 
white  in  color ;  is  partly  finely  granular  and  partly  coarsely  granular 
in  texture,  and  consists  largely  of  triclinic  feldspar,  as  is  indicated 
by  the  many  striated  cleavage  surfaces.  This  rock  is  also  musco- 


118  GEOLOGY   OF  WORCESTER. 

vitic  in  part;  and  all  of  it  has  a  shimmering  of  muscovite  or  sericite 
on  the  surfaces  of  the  folia,  which  are  parallel  to  the  sides  of  the 
bands.  In  it  may  be  found  a  light  cinnamon-colored  garnet,  and 
apatite  of  a  light  green  color  in  considerable,  though  not  well  de- 
fined, crystals.  This  rock  is  an  aplite,  and  bears  the  same  rela- 
tion to  this  granite  that  the  aplite,  in  the  dikes  of  Millstone  Hill, 
did  to  that  granite. 

Glacial  marks         On  the  surface  of  this  granite  may  be  found  glacial 
on  granite.       scratches  pointing  five  degrees  west  of  south. 

In  this  granite  is  also  found  a  rock  which  appears 
Dike  of  horn-      with  the  granite  on  west  side  of  Providence  street. 

blendic  rock 

in  granite.  as  well  as  on  the  east.  This  rock  appears  in  a 
somewhat  wide  band,  and,  at  a  glance,  is  seen  to  be 
quite  different  from  either  the  granite  or  the  aplite  already 
studied.  It  is  of  a  dark  green,  or  dark  greenish  grey,  color;  is 
fibrous,  or  finely  bladed,  in  texture;  and  has  a  foliated  structure 
because  of  the  arrangement  of  the  blades  in  parallel  planes,  pro- 
ducing an  indistinct  cleavage.  The  fresh  surface  of  the  rock  is 
marked  by  brownish  spots,  more  or  less  irregular,  and  half  an 
inch,  or  more,  in  diameter.  Even  by  the  unaided  eye  the  dark 
green,  almost  black,  blades  are  recognized  as  hornblende,  while 
the  brownish  spots  are  seen  to  consist  of  dark  brown  biotite.  So 
large  a  part  of  this  rock  is  made  up  of  these  two  minerals,  that 
we  do  riot  at  first  see  any  other,  and  may  think  it  a  massive  horn- 
blende; but  under  the  glass  we  find  a  white,  glassy  mineral,  which 
also  shows  on  the  weathered  surfaces.  This  is  a  feldspar.  This 
rock  also  constitutes  a  dike  in  the  midst  of  this  granite,  and  is 
later  than  it.  What  time  relation  the  hornblende  rock  bears  to 
the  aplite  does  not  appear,  as  the  two  rocks  were  not  seen  touch- 
ing each  other. 

This  dark   green    rock  is  closely  related   to  the 

outcrops  of       rock  appearing  in  other  outcrops  in  the  neighboring 

rockln^tgh-     fi^ds  just  east,  and  it  is  well  for  us  to  study  the 

boring  fields.      rock  of  these  outcrops  at  this  time.     One  of  these 

outcrops  may  be  found  on  either  side  of  Gibbs  street, 

five  hundred  to  five  hundred  and  fifty  feet  from  Providence  street; 

another  in  a  small,  narrow,  elliptical  area,  two  hundred  feet,  or 

so,  southwest  of  the  southwest  corner  of  the  pond  in  the  field; 

west  of  the  southern  end  of  the  same  pond  is  another  outcrop  of 

the  same  rock,  constituting  a  long,  narrow  strip,  but  a  few  feet 


PITTED  SURFACE  OF  DIORITE  FROM  BALLARD'S  FIELD,  NEAR  QUINSIGA- 
MOND.     ORIGINAL,  7  INCHES  BY  4. 


GEOLOGY    OF    WORCESTER.  119 

wide,  and  two  hundred  to  three  hundred  feet  long;  again  in  the 
next  field,  north,  is  a  circular  area  of  the  same  rock,  constituting 
a  knoll,  one  hundred  and  fifty  feet,  or  so,  in  diameter. 

In  these  different  localities,  the  rock  is  essenti- 

Outward 

appearance       ally   the   same,    though   varying   in   texture,   being 
of  the  coarser  in  one  and  finer  in  another,  and  varying  in 

diorite. 

the  relative  abundance  of  the  different  minerals. 
It  may  be  studied  to  the  best  advantage  in  the  most  northerly  of 
these  outcrops.  Here  the  rock  is  of  a  rusty  grey  color  on  the 
weathered  surface,  and  somewhat  speckled  in  appearance,  because 
of  the  difference  in  color  of  the  two  important  minerals.  It  is 
perfectly  massive,  without  sign  of  foliation ;  but  it  is  cut  by  many 
joints,  extending  in  different  directions,  into  irregular,  angular 
blocks.  In  places,  also,  the  weathered  surface  is  deeply  pitted, 
presenting  an  appearance  that  quickly  attracts  the  eye.  This  in- 
dicates that  the  minerals  in  this  rock  are  not  acted  on  and  removed 
uniformly  by  the  agents  of  the  atmosphere. 

On  attempting  to  break  this  rock  for  a  fresh  surface  to  study, 
we  find  it  exceedingly  tough ;  and  we  also  find  that  the  rusting  has 
extended  far  into  the  rock,  following  the  joints,  and  penetrating 
the  rock  from  these.  Because  of  these  facts,  and  also  on  account 
of  the  absence  of  foliation,  it  is  difficult  to  obtain  a  nicely  shaped 
specimen  presenting  fresh  surfaces. 

Within,  this  rock  is  of  a  dark  grey,  in  places, 
greenish  grey,  color;  of  crystalline,  granular  texture, 
rite  on  a  sometimes  coarse  and  sometimes  fine,  and  is  without 
surface  order  or  uniformity  in  the  arrangement  of  its  minerals. 
Under  the  magnifying  glass,  we  pick  out  the  differ- 
ent minerals.  There  are  iron  pyrites  and  magnetic  pyrites,  marked 
by  their  colors  and  lustre.  They,  in  part,  at  least,  account  for 
the  rusting  of  this  rock,  which  has  already  been  noticed.  There 
is  the  white  mineral,  some  of  it  slightly  tinted  grey,  glassy  or 
waxy  in  lustre,  and,  now  and  then,  presenting  cleavage  surfaces, 
showing  the  fine  striations  characteristic  of  a  triclinic  feldspar. 
With  ease  we  identify  the  hornblende,  bladed  in  structure,  and 
partly  dark  green,  and  partly  light  green  in  color.  Then  there 
are  the  brown  blotches,  which  are  seen  to  be  made  up  of  brown 
biotite  mica.  We  observe  that  these  blotches  are  just  about  as 
numerous  on  a  fresh  surface,  as  are  the  cavities  producing  the 
pitted  appearance  on  the  weathered  surface.  In  fact  the  biotite 


120  GEOLOGY   OF  WORCESTER. 

masses,  distributed  through  this  rock,  weather  or  decay,  under  the 
action  of  the  atmosphere,  more  rapidly  than  do  the  other  minerals, 
and  so  produce  the  pitting  on  the  weathered  surfaces.  With  much 
searching  we  may  also  find,  in  this  rock,  a  mineral, 
glassY  m  lustre,  having  a  slight  rosy  tint,  and  so 
hard  as  to  be  but  slightly,  if  at  all,  scratched  by 
the  knife  blade.  This  mineral  may  sometimes  be  found  in  par- 
ticles, even  a  half  inch  through.  This  mineral  is  garnet. 

If  we  study  this  rock  in  its  other  outcrops,  we  shall  find  it  al- 
ways practically  the  same,  composed  of  hornblende  and  triclinic 
feldspar,  with  clumps  of  brown  biotite  distributed  through  it. 
The  rock  is  a  biotite  diorite. 

But  this  diorite  is  not  limited  to  these  two  fields. 
o^theTiotfte     If  the  c°vering  of  till  were  removed  from  the  hill 
diorite  on        to  the  north  and  northeast,  we  should  find  other 
**££?*      outcrops  of  it.     Back  of  the  barn  on  the  Heywood 
farm,  which  is  on  the  north  side  of  this  hill,  may 
be  found  more  of  this  rock,  presenting  the  same  pitted  surfaces,  and 
having  the  same  composition.     The  abundance  and  wide  distribu- 
tion of   the  fragments  of   this  rock  in  the  neighboring  stone  wall 
indicate  that  there  is  a  large  area  of  it  here,  perhaps  even  larger 
than  the  area  on  the  Ballard  estate. 

Putting  together  the  different  occurrences  of  this 

Relation  of 

these  out-        diorite  in  this  field,  east  of  Providence  street  and  on  the 
biotte"dtorite     ^eywood  farm,  we  see  that  it  bears  no  fixed  relation 
to  each          to    the  neighboring  rocks.      Sometimes  it  occurs  in 
other.  schist,  and  sometimes  in  granite.     Moreover  there  is  no 

visible  connection  between  these  different  areas.  Each  one  is,  so  far 
as  we  can  see,  entirely  surrounded  by  another  kind  of  rock.  If  we 
study  this  rock  carefully,  especially  west  of  the  south  end  of  the  pond 
in  this  field,  we  shall  find  evidence  that  this  rock  is  an  eruptive  rock. 
When  in  a  molten  condition,  it  came  up  into  the  midst  of  these 
other  rocks,  filling  the  fissures,  and  there  crystallized.1  It  differs 
from  the  granite  in  that  it  is  a  basic  eruptive,  while  the  granite 
is  an  acid  eruptive.  But  while  there  may  be  no  visible  connec- 
tion between  these  different  outcrops  of  the  diorite,  there  is  prob- 
ably a  connection  beneath.  They  all  really  came  from  one  mass 
of  molten  rock,  the  larger  part  of  which  solidified  beneath  the  rocks 
which  are  now  at  the  surface.  The  garnets,  which  we  have  seen 

1  It  is  possible  that  this  diorite  is  a  continuation  of  the  Shrewsbury  dike  of  diorite 
which  was  considered  in  Chapter  V. 


GEOLOGY   OF   WORCESTER.  121 

in  the  diorite,  were  possibly  derived  from  the  neighboring  garnet- 
iferous  mica  schist,  through  which  the  molten  rock  came  into  its 
present  position. 

A  third  rock  within  this  area,  and  of  interest  for 
Gametiferous  us>  js  found  in  a  knoll  between  the  first  area  of  the 
foliated  granite  and  the  small  pond  to  the  east.  This 
rock,  on  the  weathered  surface,  is  of  a  slightly  rusty 
grey  color;  is  massive,  without  foliation  or  schist  structure,  but 
is  cut  by  joints,  in  various  directions,  into  angular,  irregular  blocks. 
The  weathered  surface  is  marked  by  irregular  spots  of  a  pink  color, 
one  half  inch  or  more  in  diameter,  the  material  of  which  stands 
out  in  relief,  indicating  unequal  weathering  of  the  surface.  These 
spots  distinguish  this  rock  from  the  neighboring  rock.  On  breaking, 
the  rock  proves  to  be  very  tough;  at  the  same  time,  the  pieces 
fly  with  great  force  under  the  blow  of  the  hammer,  making  evi- 
dent its  brittleness  and  strength.  The  rock  is  hard  and  compact, 
and  presents  a  quite  well  defined  conchoidal  fracture.  It  is  of 
a  dark  grey  color  on  the  fresh  surface,  of  medium  fine,  granular 
texture,  and  shows  nothing  of  a  schistose  structure.  There  appear, 
dotting  these  surfaces,  light  pink  glassy  blotches  one  half  inch  or 
more  in  diameter,  of  irregular,  rounded  shape,  which  are  granular 
masses  of  a  pink  garnet.  These  garnets  add  much  beauty  to 
specimens  of  this  rook. 

Under  the  magnifying  glass,  we  may  see  a  considerable  quantity 
of  dark  brown  mica,  in  bright  shining  scales,  and  also  mica  of  a 
full  black  color.  There  are  quartz  of  a  smoky  color,  and  glassy, 
colorless  feldspar,  recognized  by  its  cleavage  surfaces.  In  addi- 
tion, a  green  hornblende  particle  may,  now  and  then,  be  recognized. 
But  the  larger  part  of  the  rock  is  a  fine,  granular,  dark  grey  mix- 
ture, in  which  it  is  not  easy  to  distinguish  the  separate  minerals. 
Here  and  there,  black  mica  may  be  seen;  but  how  much  of  the 
mixture  may  be  black  hornblende  cannot  be  determined  under 
the  hand  glass. 

This  rock  is  clearly  distinguished  from  the  schistose  granite 
already  described,  situated  just  east,  by  entire  want  of  foliated 
structure,  and  difference  in  composition;  on  the  other  hand,  it 
differs  from  the  diorite  in  containing  much  quartz,  little  hornblende, 
much  more  mica,  and  many  garnets.  It  must  be  remembered 
that  we  found  in  the  diorite  a  few,  irregular,  small  masses  of  granu- 
lar garnet.  As  a  rock  this  is  best  described  by  calling  it  a  garnet- 
iferous,  hornblende,  biotite  granite;  but  if  it  is  related  to  any  of 


122  GEOLOGY    OF   WORCESTER. 

the  neighboring  rocks,  it  is  to  the  diorite  rather  than  to  the  schist- 
ose granite.  This  garnetiferous  granite  is  also  an  eruptive  rock. 
It  is  found  in  an  area  only  about  thirty  feet  in  diameter.  We 
may  see  in  it,  in  addition  to  the  minerals  already  noted,  pyrrho- 
tite,  recognized  by  its  bronzy  color,  and  metallic  lustre,  and  by 
being  attracted  by  the  magnet.  There  are,  also,  iron  pyrites  and 
some  magnetite,  the  latter  in  fine  particles,  and  found  only  by  pow- 
dering the  rock,  and  picking  out  the  particles  by  means  of  the 
magnet.  Their  color  distinguishes  them  from  the  pyrrhotite. 
These  ores  are  sufficiently  abundant  to  materially  increase  the 
specific  gravity  of  the  rock. 

By  these  eruptive  rocks,  very  interesting  in  them- 
Metamorphic  selves,  we  have  been  led  away  from  the  problem 
Baiiard  field.  which  was  presented  to  us,  —  the  relation  of  the  Bolton 
gneiss  to  the  Carboniferous  rocks  already  studied 
in  previous  chapters.  These  eruptives  do  not  help  us  at  all  in 
solving  the  problem:  they  tend,  by  their  presence  and  effect  on 
the  neighboring  rock,  to  make  this  solution  more  difficult.  We 
must  now  study  the  rocks  of  this  area  that  were  here  before 
either  granite  or  diorite  came  into  their  midst. 

There  are  two  of  these  rocks.     The  first,  nearer 

The  mica  .  . 

schist  of  Providence  street,  appears  in  many  outcrops.  It  is 
Baiiard  a  mjca  schist.  In  some  outcrops  it  is  exceedingly 
rusty;  and  the  iron  pyrites,  producing  this  effect, 
may  frequently  be  seen  within  the  fresh  rock.  Where  not  rusty, 
the  rock  is  of  a  light  grey  color;  in  part,  it  is  thinly  fissile,  and,  in 
part,  is  quite  massive,  or  appears  so,  before  breaking.  This  mas- 
sive variety  is  seen  at  the  corner  of  Gibbs  and  Providence  streets. 
It  is  a  light  grey  mica  schist,  almost  silvery  in  lustre,  containing 
only  a  little  biotite.  In  its  different  outcrops,  it  contains  various 
minerals,  as  pink  and  yellowish  garnets,  fibrolite,  and  andalusite. 
In  places,  also,  graphite  gives  to  this  rock  a  dark  grey  color  and 
noticeably  greasy  feel.  This  schist  was  made  out  of  ancient  sed- 
iments, and  is  older  than  the  eruptives  already  described.  They 
were  forced  from  beneath  into  its  midst.  This  schist  is,  then, 
one  of  the  connecting  links  between  the  Bolton  gneiss  on  the 
east  and  the  Carboniferous  rocks  on  the  west. 

In  going  across  this  field  to  the  east,  we  encounter 

Micaceous 

quartzite        still  another  rock  at  the  foot  of  the  small  pond. 


in  Baiiard        This  is  of  a  medium  fine,  granular  texture,  of  grey, 
or  rusty  grey,   color,   and  of  laminated  structure. 


GEOLOGY    OF   WORCESTER.  123 

Under  the  glass  a  part  of  the  laminae  are  seen  to  be  composed, 
largely,  of  glassy,  granular  quartz,  slightly  rusty;  the  remainder 
of  the  laminae  contain  much,  somewhat  coarse,  black,  or  dark 
brown,  mica,  causing  these  laminae  to  be  much  darker  than  the 
others.  In  addition  there  is  some  feldspar  in  these  laminae,  as 
is  indicated  by  the  dull  white  color  of  some  particles  on  the  wea- 
thered surfaces.  This  rock  is  also  garnetiferous.  These  alter- 
nating light  and  dark  laminae,  frequently  only  a  small  fraction 
of  an  inch  thick,  are  not  arranged  regularly  with  the  general  dip 
and  strike  of  the  rock,  but  are  intricately  folded,  crumpled,  and 
faulted,  so  that  a  single  lamina  cannot  be  traced  with  certainty 
more  than  a  short  distance.  Because  of  this,  the  dip  and  strike 
are  not  easily  determined.  This  rock,  from  its  constituent  min- 
erals, may  be  called  a  micaceous  quartzite.  It  may  be  traced, 
by  outcrops,  to  the  north  and  south  for  a  short  distance.  It 
constitutes  a  narrow  band,  one  hundred  to  two  hundred  feet  wide, 
east  of  the  schist  just  studied.  Within  the  micaceous  quartzite 
may  be  found  narrow  dikes  of  granite,  which  is  partly  fine,  and 
partly  coarse,  grained.  This  quartzite  does  not  extend  east  as 
far  as  the  Bolton  gneiss. 

Mica  schist  Between   the  last  rock   studied   and   the   Bolton 

between  Boi-      gneiss  is  more  mica  schist.     This  is  also  of  a  light 
tona^eiss        grey  color,  almost  silvery,  and,  in  places,  somewhat 
micaceous        rusty;    is  generally  thinly  laminated,  but,  in  part, 
zlte'        is  apparently  massive;    has  something  of  a  smooth, 
soapy  feel ;  abounds  in  garnets,  and  is  sometimes  fibrolitic ;  and  it 
contains  a  light  grey  mica,  with  but  little  biotite.     On  placing  spec- 
imens of  this  schist,  side  by  side,  with  specimens  from  the  schist 
west  of  the  micaceous  quartzite,  we  can  see  no  difference.     We 
are  forced  to  believe  these  schists,  east  and  west  of  the  micaceous 
quartzite,  one  and  the  same.     This  schist  extends  east  to  the 
Bolton  gneiss.     The  data  that  we  must  use  in  the  solution  of  our 
original  problem  are  before  us. 

In  considering  this  we  are  led  to  ask:  What  relation  does  the 
micaceous  quartzite  bear  to  the  schist  in  whose  midst  it  occurs? 
To  answer  this  question,  we  must  carefully  examine  the  contact, 
or  the  line,  where  the  quartzite  joins  the  schist  on  the  east  and 
on  the  west.  Fortunately  for  our  study  these  lines  are  fairly 
well  exposed  in  the  most  southern  part  of  the  field,  near  Gibbs 
street.  There  we  may  see  the  quartzite  laminae  dipping  west, 


124  GEOLOGY    OF   WORCESTER. 

and  down,  under  the  laminae  of  the  schist;  at  the  eastern  contact 
we  likewise  see  the  laminae  of  the  schist  dipping  down  under  the 
quartzite.  In  one  case  quartzite  dips  beneath  schist;  in  the 
other,  schist  dips  beneath  quartzite.  These  two  facts  of  them- 
selves do  not  help  us  in  determining  which  is  the  upper  and 
which  is  the  lower  formation,  but  tell  us  that  there  is  here  a 
compressed  and  overturned  fold. 

To  make  clear  the  last  sentence,  let  us  consider 

tfeunfefteMws     Figure  I.1     There  is  a  compressed  synclinal  fold  on 

quartzite  to       the  left,   and  a   compressed  anticlinal  fold  on  the 

t^ch^tCa  right,  and  both  are  tipped  over  to  the  right.  One 
side  of  each  fold  is  parallel  with  the  other  side  of 
the  same  fold,  and  these  sides  differ  in  direction  only  at  the  top 
and  bottom  of  the  fold  where  the  sides  curve  towards  each  other. 
There  are  included  in  these  two  folds  two  different  bands  distin- 
guished by  the  markings.  That  marked  1-2,  3-6  and  4-6  is  one 
band,  and,  if  the  folds  were  unfolded  horizontally  across  the  page, 
that  band  would  be  above  the  other  marked  2-7,  7-3  and  4-5. 
That  the  former  band  is  the  upper  is  clear  even  in  the  folds,  because 
we  see  the  whole  of  the  folds. 

Figure  2  is  simply  a  reproduction  of  the  lower  part  of  Fig.  1, 
with  all  above  the  line  A  B  in  Fig.  1  omitted.  In  this  figure  it 
is  easy  to  see  that  the  band  1-2,  3-6,  6-4  is  the  upper  band,  because 
we  see  the  bottom  of  the  left  hand  fold,  and  can  easily  supply  from 
the  figure  above  the  part  of  the  curve  that  is  lacking.  If  now, 
as  in  Fig.  3,  we  reproduce  only  a  section  from  Fig.  2,  omitting 
the  lower  and  upper  part  of  the  curve,  we  can  readily  see,  by  com- 
paring it  with  the  other  two  figures,  exactly  what  part  of  those 
figures  is  reproduced  and  that  marked  1-2,  6-3  is  the  upper  band. 

But  if  Figures  1  and  2  were  not  on  the  page  to  indicate  the  re- 
lation of  the  bands  1-b  and  3-d,  it  is  evident  that  the  bands  might 
be  joined  below  by  continuing  and  curving  the  boundary  la  around 
to  join  6d,  and  2b  to  join  3c.  This  figure  would  then  represent 
a  compressed  synclinal  fold.  In  that  case  the  band  1-b,  6-c 
would  be  the  lower  band,  and  the  band  7-b,  7-c  would  be  the 
upper.  If,  on  the  other  hand,  the  boundaries  a-1,  b-2  are  con- 
tinued above  the  figure  and  curved  to  join,  respectively,  the  boun- 
daries d-6,  c-3,  the  bands  will  then  constitute  an  anticlinal  com- 
pressed fold.  In  this  case  the  band  1-b,  6-c  would  be  the  upper 
band  resting  on  the  band  7-b,  7-c. 

1  Page  126. 


GEOLOGY    OF    WORCESTER. 


125 


A 


126  GEOLOGY   OF  WORCESTER. 

From  this  discussion,  it  is  seen  that  the  bands  1-b,  6-c  become 
the  lower  or  upper  band  according  to  whether  they  are  joined 
beneath  into  a  synclinal  or  above  into  an  anticlinal  fold.  There 
is  nothing  in  Fig.  3  to  tell  us  which  is  the  correct  drawing,  if  this 
figure  is  considered  independently  of  Figs.  1  and  2.  Let  it  be 
noticed,  also,  in  Fig.  3  that  the  band  1-b  dips  under  the  band 
7-b ;  while  the  band  6-c,  corresponding  to  1-b,  leans  against  7-c 
corresponding  to  7-b.  In  one  case  the  first  band  dips  under  the 
second;  in  the  other  the  second  dips  under  the  first.  From  this 
it  is  evident  that  their  relative  positions  tell  us  nothing  as  to  which 
may  really  be  the  upper  or  the  lower  band. 

Let  us  now  compare  the  conditions  presented  in  Fig.  3  with 
the  facts  presented  by  the  mica  schist  and  quartzite  in  the  field 
just  north  of  Gibbs  street.  We  must  first  bear  in  mind  that  this 
land  surface  has  been  subjected  to  great  erosion  so  that  it  is  made 
up  of  only  remnants  of  formations.  We  must  not  expect  facts  to  be 
presented  in  their  entirety.  On  the  west  the  micaceous  quartz 
dips  beneath  the  mica  schist,  and  on  the  east  the  schist  dips  be- 
neath the  quartzite.  We  cannot  see  beneath  the  surface  to  deter- 
mine whether  or  no  the  schist  extends  down  below  the  quartzite 
and  joins  the  schist  on  the  other  side;  neither  can  we  determine  from 
what  remains  whether  or  no  the  schist  bands  formerly  joined  above 
the  quartzite.  Our  only  hope  in  such  compressed  folds  is  in  finding 
the  end  of  the  fold  where  we  may  see  the  curve  in  the  beds  by  which 
we  shall  know  whether  the  beds  curve  down  in  a  syncline  or  up 
in  an  anticline.  As  the  mica  schist  on  either  side  does  not  help 
us,  the  micaceous  quartzite  must  be  carefully  studied.  Unfortu- 
nately for  this  study,  its  laminae  have  been  so  crumpled  and 
faulted  in  this  ledge,  that  a  single  lamina  can  with  difficulty  be 
traced  only  a  few  feet  at  the  most,  to  say  nothing  of  tracing 
the  same  lamina  from  one  side  of  the  outcrop  to  the  other.  We 
are  forced  to  abandon  our  study  at  this  ledge,  but  we  have  clearly 
in  mind,  from  the  study  made  here,  exactly  that  for  which  we 
must  look.  Let  us,  then,  examine  the  next  outcrop,  north,  of 
the  quartzite,  which  presents  to  us  the  rock  surface  represented 
in  the  opposite  picture.  This  shows  us  clearly  the  centre  of  an 
anticlinal  fold.  But  the  difficulty  with  this  is  that  we  cannot  be 
certain  that  it  is  the  centre  of  a  large  anticlinal  fold  including  the 
whole  of  the  quartzite.  It  may  be  only  a  small  anticline,  con- 
stituting a  secondary  fold  on  either  a  large  anticline  or  a  large 


AN  ANTICLINE  IN   MICACEOUS    QUABTZITE  IN    BALLARD'S    FIELD.    NEAR 
QUINSIGAMOND. 


CRUMPLED     LAMINAE     OF     THE     MICACEOUS     QUARTZITE,     FOUND     AT 

QUINSIGAMOND. 


GEOLOGY    OF    WORCESTER.  127 

syncline.  This  is,  however,  the  best  that  we  can  find  in  this  out- 
crop, because  the  crumpling  and  faulting  have  been  so  severe  as 
to  obliterate,  conceal  or  confuse  the  lines  of  lamination. 

Keeping  this  small  anticline  in  mind,  as  a  starting  point,  we  go 
still  farther  north  to  the  outcrop  south  of  the  pond,  where  we 
have  already  studied.  After  careful  study  and  examination  of  the 
southern  end  of  this  outcrop,  we  may  see  laminae  of  the  east 
side  of  this  outcrop  slanting  or  dipping  easterly;  on  the  west 
side  of  the  same  outcrop  the  laminae  are  dipping  westerly. 
Going  around  to  the  northern  side  of  this  outcrop,  we  may  there 
see  the  laminae  arching  over  from  west  to  east  in  an  anticlinal 
fold,  whose  top  slants  down  to  the  north.  The  geologist  expresses 
this  by  saying  that  the  fold  pitches  north.  Here,  fortunately  for 
our  study,  the  fold  is  not  compressed,  as  is  the  fold  a  little  farther 
south,  but  is  here  an  open,  anticlinal  fold.  Having  found  that 
this  quartzite  is  in  the  form  of  an  anticline,  we  are  now  able  to 
understand  the  relation  it  bears  to  the  schist  occurring  on  either 
side  of  it.  The  quartzite  extends  beneath  the  schist  and  was 
folded  into  an  arch  up  into  the  midst  of  the  schist;  then  as  erosion 
lowered  the  surface  of  the  earth  by  removing  the  rocks  above, 
this  quartzite,  in  the  form  of  an  arch,  came  to  be  exposed  with 
the  schist  on  either  side  of  it.  The  quartzite  is  therefore  beneath, 
and  is  the  older  rock  of  these  two. 

But  this  micaceous  quartzite  is  a  well  defined  rock, 
and  micaceous  and  one  that  is  not  easilv  mistaken.  We  have  al- 
quartzite  ready  become  well  acquainted  with  it  in  Wigwam 
iferous  Hill.  Between  these  two  localities,  there  are  inter- 
vening outcrops,  showing  clearly  that  the  two  are 
one  rock,  and  connected.  From  Wigwam  Hill  we  may  directly 
trace,  by  a  series  of  outcrops,  this  folded  micaceous  quartzite  to 
the  deep  cut  of  the  B.  &  A.  R.  R.,  at  the  bridge  where  Plantation 
street  crosses  the  railroad.  Here  this  same  micaceous  quartzite 
appears  in  the  midst  of  the  Carboniferous  phyllite.  Here  the 
quartzite  is  folded  or  faulted  up  into  the  Carboniferous  phyllite 
in  a  manner,  probably,  similar  to  that  in  which  the  quartzite  is 
folded  up  into  the  mica  schist  in  the  field  on  the  Ballard  estate 
at  Quinsigamond.  In  other  words  the  micaceous  quartzite  at 
Quinsigamond  is  the  Carboniferous  quartzite,  and  the  mica  schist 
on  either  side  of  it,  being  above  it,  is  really  only  the  Carboniferous 
phyllite  in  a  little  more  highly  metamorphosed  condition. 


128  GEOLOGY   OF   WORCESTER. 

That  this  conclusion  is  correct,  is  made  clear  by  finding, 
in  this  silvery  schist  in  this  field  at  Quinsigamond,  small  areas 
of  schist  that  cannot  be  distinguished  from  the  regular  Carbon- 
iferous phyllite,  as  it  is  found  at  the  deep  cut  of  the  B.  &  A.  R. 
R.  There  is  a  very  thin  strip  of  regular  Carboniferous  phyllite  in- 
cluded in  the  schistose  granite  by  the  side  of  Providence  street,  and 
this  is  on  the  side  of  the  granite  towards  the  silvery  schist  that  we 
have  been  studying,  and  very  close  to  the  latter.  Just  east  of  the 
garnetiferous  granite  is  a  graphitic  phyllite  so  closely  resembling 
much  of  the  regular  Carboniferous,  that  the  two  cannot  be  dis- 
tinguished; again,  on  the  very  border  of  the  Bolton  gneiss,  is  more 
of  the  schist  that  is  not  to  be  distinguished  from  the  Carboniferous. 
To  repeat,  for  the  sake  of  emphasis,  we  may  conclude  that  the 
light  grey,  somewhat  rusty,  mica  schist,  between  what  we  had 
before  identified  as  Carboniferous  and  the  Bolton  gneiss,  is  also 
Carboniferous,  metamorphosed  one  degree  more  highly  by  the 
granite  within  its  midst,  and  by  the  granite  possibly  beneath  it. 
We  have  thus  traced  the  Carboniferous  formation  to  the  very 
border  of  the  Bolton  gneiss. 

It  now  remains  for  us  to  consider  the  relation  of 
Boiton  gneiss  the  Carboniferous  schist  to  the  Bolton  gneiss,  and 
to  carbonif-  tnus  determine  the  position  of  the  latter  in  the  his- 

erous  rocks.  . 

torical  series.  The  best  place  for  our  study  is  at  the 
head  of  this  gulch,  which  Gibbs  street  crosses  by  means  of  a  stone 
bridge,  for  there  are  many  outcrops  where  these  two  rocks  approach 
each  other.  Starting  with  outcrops  west  of  the  head  of  the  gulch, 
which  are  clearly  of  the  light  grey,  silvery  Carboniferous  mica  schist, 
we  go  east,  constantly  breaking  off  specimens  for  study,  and  at  the 
same  time  noting  the  direction  and  dip  of  the  laminae.  After  pass- 
ing a  few  hundred  feet  east  of  the  gulch,  we  notice  that  the  rock  has 
now  come  to  be  largely  the  gneissoid  granite,  which  was  so  abundant 
at  the  Ballard  quarry.  We  are  now  on  the  Bolton  gneiss.  At  some 
point  we  crossed  the  line  between  the  two.  In  passing  from  one  to 
the  other  no  change  is  noticed  in  the  dip  or  the  strike.  The  laminae 
of  one  are  parallel  with  those  of  the  other.  There  is,  however, 
a  great  increase  noticed  in  the  quantity  of  granite  that  has  been 
injected  between  the  laminae,  as  soon  as  we  get  on  the  gneiss. 
While  a  little  granite  may  be  seen  in  both  the  silvery,  grey  schist 
and  in  the  micaceous  quartzite  to  the  west,  here  in  the  Bolton 
gneiss  fully  one  half  of  the  rock  is  granite.  This  granite  is  foliated 


GEOLOGY    OF    WORCESTER.  129 

parallelly  with  the  laminae  of  the  schist  bands  inclosing  it,  and 
the  bands  of  each  show  a  crumpling  in  common.  But  let  us  com- 
pare the  grey,  silvery  schist,  west  of  the  gulch,  with  the  schist  of 
the  bands  alternating  with  the  granite  in  the  Bolton  gneiss.  The 
former  is  highly  muscovitic,  with  a  very  small  proportion  of  granu- 
lar quartz;  while  the  latter  is  biotitic,  and  contains  a  much  larger 
proportion  of  granular  quartz.  These  schists  are  not  the  same; 
yet  the  uniformity  of  dip  and  strike  points  to  a  close  relationship. 
In  comparing  different  specimens  we  may,  per- 

comparing  chance,  compare  some  of  the  specimens  of  the  biotite 
Boiton  gneiss  schist  from  the  Bolton  gneiss,  near  the  head  of  the 
with  the  mica-  gulch,  with  specimens  of  the  micaceous  quartzite  at 

ous_quar  -        ^e  foot  of  the  poj^  a  ijttie  to  the  wegt      A  marked 


resemblance  is  immediately  seen.  Each  is  abundantly 
biotitic  ;  each  is  very  sandy,  or  made  up  largely  of  glassy,  granular 
quartz;  each  is  made  up  of  the  thin  light  and  dark  alternating 
bands,  severely  crumpled.  The  only  difference  between  the  two 
is  that  the  schist  from  the  Bolton  gneiss  has  more  granite  mate- 
rial soaked  into  it.  We  therefore  conclude,  after  a  careful  study, 
that  the  alternating  schist  bands  of  the  Bolton  gneiss  are  simply 
bands  of  the  micaceous  quartzite,  a  little  more  highly  metamorph- 
osed through  the  influence  of  the  large  proportion  of  injected 
granite  between  the  laminae.  But  the  micaceous  quartzite  has 
already  been  traced  into  such  close  relationship  with  the  Carbonif- 
erous schist  or  phyllite  that  the  quartzite  can  only  be  considered 
as  Carboniferous.  The  schist  of  the  Bolton  gneiss,  being  the  same 
as  the  micaceous  quartzite,  must  then  also  be  considered  as  Car- 
boniferous. In  the  historical  series  the  Bolton  gneiss  constitutes 
a  part  of  the  Carboniferous  of  Central  Massachusetts.  It  is  the 
micaceous  quartzite  made  gneissoid  by  the  abundant  injection  of 
granite  between  its  laminae. 


10 


CHAPTER  VII. 
PAXTON  AND  BRIMFIELD  SCHISTS. 

Having  now  carried  our  study  of  the  rock-floor  to 
the  eastern  boundary  of  Worcester  and  beyond,  let 
us  retrace  our  steps,  and  go  to  the  west.  We  have  already  studied 
the  Carboniferous  quartzite  as  it  occurs  in  Pleasant  street  near 
Newton  square,  and  at  the  foot  of  Chadwick  street;  and  we  natural- 
ly inquire  in  regard  to  the  western  extension  of  this  formation. 
Let  us  then  go  out  through  Pleasant  street  towards  Tatnuck,  being 
constantly  on  the  watch  for  outcropping  ledges  to  tell  us  of  the 
underlying  rock-floor.  Unfortunately  for  us  in  this  study  and 
search,  the  drumlins  are  very  abundant  in  this  area,  and  effect- 
ually conceal  the  underlying  rock.  We  do  not  find  any  ledge 
until  we  have  passed  Tatnuck,  and  have  begun  to  ascend  the  hill 
on  the  way  to  Paxton.  Just  before  reaching  the  watering  trough, 
after  crossing  Tatnuck  brook,  we  notice  on  the  left,  or  south  side 
of  the  road,  a  good  outcrop.  Here  we  find  two  kinds  of  rock  in 
the  same  ledge.  The  one  is  a  coarse,  crystalline  rock, 
more  or  less  rusty  in  color.  In  this  we  recognize  the 
coarse  feldspar  particles  by  their  cleavage  surfaces 
and  their  porcelain-like  lustre,  and  the  quartz  by  its  glassy  lustre, 
want  of  cleavage,  and  its  hardness;  and,  distributed  through  this 
mixture  of  feldspar  and  quartz,  we  recognize  black  prisms  of  tour- 
maline by  their  resinous  lustre  and  shape.  This  part  of  the  ledge 
is  a  tourmaline  granite;  and,  as  in  the  case  of  the  other  granites 
already  studied,  is  later  or  newer  than  the  rock  in  which  it  is 
included,  and  into  which  it  was  injected.  This  other  rock  gives 
Quartzose  evidence  of  having  been  disturbed  as  the  granite  was 
mica  schist.  forced  into  its  midst,  for  its  layers  or  laminae  have 
been  bent  out  of  their  normal  position  so  as  to  make  room  for  the 
granite  between  them.  Hence,  as  we  take  the  strike,  we  find  a  con- 
siderable variation  in  the  direction  of  the  laminae,  as  they  curve 
around  the  irregular  granite  masses.  The  strike  varies  from  eight 
degrees  east  of  north  to  fifteen  degrees  west  of  north;  while  the  dip 


BANDED  PAXTON  SCHIST.    SIZE  OF  ORIGINAL,  (5  INCHES  BY  5. 


GEOLOGY    OF    WORCESTER.  131 

is  from  fifty  to  fifty-five  degrees  towards  the  west.  This  laminated 
rock  is  the  original  rock,  and  is  worthy  of  careful  study;  but  as  there 
is  not  much  of  it  here,  and  as  it  is  more  or  less  weathered,  and  we 
may  find  difficulty  in  obtaining  good  specimens  for  study,  let  us 
delay  the  careful  study  of  it  until  we  reach  a  quarry  in  this  same 
rock,  somewhat  recently  opened,  farther  up  on  this  hill. 

This  quarry  is  also  on  the  south  side  of  the  road, 
raquarfyhlU      and  is  a  short  distance  west  of  the  road  leading  from 

the  Paxton  road  to  the  Tatnuck  Country  Club  house. 
The  first  fact  to  attract  our  attention,  as  we  enter  the  quarry,  is 
the  perfection  of  the  lamination.  On  account  of  this  the  rock 
breaks  into  thin  slabs,  especially  near  the  surface  of  the  ledge. 
These  slabs  are  frequently  half  an  inch,  or  even  less,  in  thickness, 
but  as  the  depth  increases,  the  slabs  become  thicker,  so  that  those 
removed  from  the  bottom  of  the  quarry  may  be  a  few  feet  in  thick- 
ness. From  this  we  conclude  that  the  final  separation  of  this  rock 
into  very  thin  slabs  is  due  to  frost  and  other  weathering  agents, 
acting  along  planes  of  weakness  that  are  already  in  the  rock.  Ex- 
tending nearly  vertically  and  easterly  and  westerly,  and  also  north- 
erly and  southerly,  are  two  sets  of  cross- joints  which  may  be 
distinctly  traced  in  different  parts  of  the  quarry.  These  joints 
cross  the  planes  of  lamination  obliquely,  and  give  to  the  slabs  a 
rhombohedral  form.  So  smooth  and  well-defined  are  these  surfaces, 
that  they  remind  us  of  those  produced  by  crystalline  cleavage. 
The  lamination  planes  result  from  the  arrangement  of  crystalline 
particles;  and  the  separation  of  the  rock-mass  into  sheet  units 
is  due  to  the  original  layers  in  which  this  material  was  deposited 
by  water  before  it  was  solidified  or  cemented.  The  cross-joints  are 
due  to  tension  within  the  rock-mass,  probably  produced  by  the 
folding  to  which  this  rock  has  been  subjected. 

But  let  us  study  the  rock  itself,  for  here  is  an  ex- 

cellent  opportunity  on  account  of  the  freshness  of 
schist  of  the  ^he  specimens.  A  large  part  of  the  rock  of  the  quarry 

is  of  a  brownish  grey  color,  with  here  and  there  a 
greenish  tinge,  having  an  indistinct  banding  generally  parallel  to 
the  lamination.  It  has  a  medium  fine,  granular  texture.  Ex- 
amining it  under  the  glass,  we  see  that  the  brownish  tint  is  due  to 
brownish  mica,  distributed  abundantly  in  fine  scales  parallel  to 
each  other  and  to  the  lamination  of  the  rock.  The  splitting  of 
the  rock  is  due,  in  part  at  least,  to  this  arrangement  of  these  mica 


132  GEOLOGY   OF  WORCESTER. 

scales.  The  remainder  of  this  rock  is  a  finely  granular  mass  of 
glassy  particles,  some  of  which  glisten  with  cleavage  surfaces  indi- 
cating feldspar,  while  others  are  simply  particles  of  quartz.  In 
addition  to  these  we  find  distributed  irregularly  and  in  clumps, 
and  associated  with  small,  thin  masses  of  a  mixture  of  feldspar 
and  quartz,  hornblende,  sometimes  of  a  light  green  color,  and 
sometimes  of  a  dark  green,  sometimes  granular,  and  sometimes 
in  small  bladed  crystals.  This  rock  is,  then,  largely  a  mixture 
of  fine  granular  quartz  and  brown  mica;  it  is  a  mica  schist, 
variation  A  considerable  part  of  this  schist  has  a  marked 

in  Paxton  banded  appearance.  This  banding  is  due  to  alter- 
nating layers  of  brownish  and  greenish  grey,  the 
greenish  bands  being  much  lighter  in  color.  These  bands  are 
frequently  a  quarter  of  an  inch  or  less  in  thickness,  at  other 
times  an  inch  or  even  more.  The  dividing  lines  between  these 
bands  are,  in  some  cases,  clear-cut  and  sharply  defined;  in  other 
cases,  the  bands  blend  into  each  other.  The  brownish  bands 
are  identical  with  the  schist  already  described;  the  green- 
ish bands  are  lighter  from  the  absence  of  the  brown  mica,  and 
have,  in  place  of  it,  a  fine  green  hornblende.  On  searching  these 
greenish  bands  carefully,  we  observe  little,  amber  colored  crystals 
distributed,  here  and  there,  but  most  abundantly  with  the  coarse, 
dark  green  hornblende.  These  amber  colored  particles  are  prob- 
ably little  garnets.  In  other  respects  the  greenish  bands  are  like 
the  brownish,  and  may  be  called  bands  of  hornblende  schist.  This 
banding  of  the  rock,  with  the  abundance  of  quartz  in  it,  tells  us 
of  the  former  condition  of  this  rock,  when  it  was  in  the  form  of 
impure  sands  deposited  by  the  waters  of  an  ancient  sea.  The 
brownish  bands  were  layers  which  contained  a  little  more  clay,  and 
the  green,  or  hornblendic,  bands  contained  more  iron  rust.  Since 
the  time  when  they  were  layers  of  sand  deposited  along  the  shores 
of  an  ancient  ocean,  they  have  been  buried  deeply  in  the  earth, 
and  recrystallized,  just  as  the  rocks  in  the  Quinsigamond  quarry 
have  been  recrystallized.  Because  of  the  abundance  of  this  mix- 
ture of  mica  and  hornblende  schists  in  the  ledges  of  the  neighboring 
town  of  Paxton  the  mixture  may  be  called  the  Paxton  schist. 

Tourmaline  But    tO    retum    tO    ^6    stucty    of    the    rock    of    this 

granite  in        quarry — we   find   still   another   rock.     It   is   nearly 
Tatnuck  hiii      white   in   color,   and   hence   attracts   our  attention. 

quarry.  ..... 

It  occurs  without  any  regularity  in  position,  and  in 


TOURMALINE  GRANITE  IN  PAXTON  SCHIST.    SIZE  OF  ORIGINAL  SPECIMEN, 
6  INCHES  BY  4.    FROM  QUARRY  SOUTH  OF  PAXTON  ROAD. 


GEOLOGY    OF    WORCESTER.  133 

more  or  less  nodular-like  dikes  or  masses,  cutting  across  the  bands 
of  the  schist.  At  other  times  the  bands  of  the  schist  wrap  around 
the  nodular  masses.  In  other  words  there  is  no  fixed  relation 
which  the  white  rock  and  the  schist  bear  to  each  other.  The 
masses  of  the  white  rock  vary  greatly  in  size — sometimes  a  foot, 
sometimes  several  feet,  through;  sometimes  these  masses  are  con- 
nected, sometimes  separate  and  distinct.  We  examine  the  rock 
itself.  It  is  sometimes  fine  and  sometimes  coarse.  The  same  mass 
frequently  shows  this  variation  from  fine  to  coarse,  being  fine 
around  the  border,  where  in  contact  with  the  schist,  and  coarse 
in  the  centre.  From  the  manner  in  which  the  rock  occurs,  and 
from  the  minerals  which  it  contains,  we  recognize  it  as  a  granite. 
It  was  evidently  forced  or  intruded  into  the  midst  of  the  schist, 
while  the  schist  was  in  a  yielding  condition,  for  the  schist  frequent- 
ly folded  and  bent  without  fracture;  and,  even  when  the  granite 
cuts  across  the  laminae  of  the  schist,  the  break  was  more  like  that 
in  a  soft,  pasty  substance,  than  that  in  a  hard,  brittle  mass.  The 
intrusion  of  the  granite  may  be  associated  with  the  period  of  re- 
crystallization  of  the  schist,  though  the  variation  in  the  size  of 
the  grains  of  the  granite  indicates  that  the  granite  came  in  contact 
with  surfaces  somewhat  cool,  producing  a  more  rapid  crystalliza- 
tion around  the  border,  and  hence  finer  granular  texture  next 
to  the  schist. 

In  this  granite  we  recognize  the  feldspar  by  its  pearly  white 
cleavage  surfaces.  This  is  nearly  all  orthoclase  feldspar,  for  it  is 
difficult  to  find  a  striated  surface  indicating  a  triclinic  feldspar. 
The  coarser  feldspar  is  frequently  not  pure  white,  but  of  a  slightly 
greyish  tint.  The  quartz  is  also  white,  but  is  more  glassy,  and 
lacks  the  smooth  cleavage  surfaces.  Besides  these  minerals  this 
granite  uniformly  contains  black,  columnar  crystals  of  tourmaline, 
the  prisms  varying  considerably  in  size,  and  showing  the  charac- 
teristic resinous  lustre  and  grooved  sides.  The  almost  complete 
absence  of  mica  in  this  granite  attracts  our  attention;  the  tour- 
maline seems  to  take  its  place.  We  also  note  that  frequently  the 
tourmaline  is  not  uniformly  distributed,  but  is  condensed  around 
the  border  next  to  the  schist.  Because  of  the  abundance  of  the 
tourmaline,  this  granite  may  rightly  be  called  a  tourmaline  granite. 
Garnetiferous  ^n  some  °f  this  granite,  especially  the  finer  grained, 
tourmaline  we  may  find  little,  light  pink  garnets.  The  faces  on 
granite.  these  crystals  are  not  clearly  marked,  so  we  cannot 


134  GEOLOGY   OF   WORCESTER. 

make  out  the  exact  mathematical  form   of  the   crystals.     This 
granite,  with  its  tourmaline  and  garnets,  marks  a  distinct  variety 
not  before  met  with  in  our  study  of  the  rocks  of  Worcester,  and 
we  take  a  specimen  of  it  to  represent  this  variety  of  rock. 
Edge  of  the  Before  leaving  the  quarry,  we  must  observe  the 

Pl<Strai°f       dip  and  strike  of  the  laminae  of  the  schist,  to  help 
Massachu-        us  in  placing  it  in  its  proper  position  with  reference 
setts.  t^  the  other  rocks  studied  in  the  rock-floor.     We 

find  the  laminae  striking  or  pointing  almost  exactly  north  and 
south,  and  dipping  about  thirty  degrees  to  the  west.  In  other 
words,  as  we  ascend  this  hill,  which  is  really  the  edge  of  the  pla- 
teau of  Central  Massachusetts,  we  are  walking  up,  or  across,  the 
edges  of  these  upturned  laminae  which  are  sloping  down  into  the 
hill  to  the  west.  The  strike  may  be  best  taken  up  on  the  surface 
of  the  ledge  where  the  rock  has  not  been  quarried. 

Here,  also,  we  may  see  the  glacial  scratches  nicely 
marks  preserved  on  the  surfaces  of  the  granite  masses  that 

are  imbedded  in  the  schist.  Undoubtedly  these 
scratches  were  made  over  the  whole  surface,  on  the  schist  as  well 
as  on  the  granite,  but  the  weathering  or  crumbling  of  the  surface 
of  the  schist  has  been  sufficient  to  remove  them.  We  also  take 
their  direction  and  find  that  they  point  five  to  ten  degrees  east 
of  south.  Comparing  this  direction  with  those  found  in  the  eastern 
part  of  the  city  we  may  conclude  that  the  ice  of  the  Glacial  Period 
moved  in  an  almost  southerly  direction  with  a  slight  deviation 
to  the  east. 

other  min-  ^n  Edition  to  the  minerals  already  mentioned, 

erais  in  this       there  may  be  found  in  this  quarry  iron  pyrites  in 

fine  particles,  in  both  the  schist  and  the  granite; 

the  surface  of  one  slab  was  covered  with  a  thin  coating  of  stilbite 

crystals.     The  indications  are  that  this  quarry  will  afford  but 

few  minerals,  and  will  not  compare  in  this  respect  with  the  quarry 

at  Quinsigamond. 

As  the  laminae  of  the  Paxton  schist  of  the  quarry  point  nearly 
north  and  south,  we  may  expect  to  find  it  extending  in  those  di- 
rections ;  and  so  we  cross  the  fields  to  the  south  seeking  for  traces 
of  it.  We  find  it  in  outcrops  north  of  the  next  road,  Fowler  street, 
leading  from  Tatnuck  up  the  edge  of  the  plateau  to  the  Lynde 
Brook  Reservoir.  The  same  rock  appears  in  numerous  outcrops 
on  the  side  hill  south  of  this  road.  There  the  laminae  strike  nearly 


GEOLOGY    OF   WORCESTER.  135 

north  and  south,  deviating  only  five  to  ten  degrees  east  of  north, 
and  dip  forty  degrees  to  the  west. 

As  we  follow  Fowler  street  up  the  hill,  near  the 
Granite  area      top  on  tne  east  siOpe   we  notice,  on  the  south  side 

south  of  r 

Fowier  street,  of  the  road,  and  in  the  road,  a  rock  constituting 
a  ledge,  yet  evidently  not  the  schist  which  we  are 
following.  From  its  general  resemblance  to  the  granite  already 
seen  at  the  quarry,  we  may  surmise  that  it  is  more  granite  within 
the  schist.  Careful  study  confirms  this.  As  in  other  cases,  we 
break  off  several  pieces  for  examination.  This  rock  is  of  a  grey 
color,  and  of  a  medium  fine,  granular  texture.  It  has  a  noticeable 
foliation  parallel  to  the  lamination  of  the  neighboring  schist. 
There  are  many  glistening  scales  of  white  mica  or  muscovite,  of 
darker  mica  of  a  brownish  color,  and  of  black  mica  or  biotite.  There 
are,  then,  two,  if  not  three,  kinds  of  mica  present  in  this  rock. 
The  quartz  and  feldspar  generally  make  up  a  fine  granular,  sugary 
mixture,  in  which  may  be  distinguished  with  difficulty  the  two 
minerals;  but,  here  and  there,  a  coarser  particle,  now  of  glassy 
quartz  and  now  of  feldspar,  appears.  By  examining  several  of  the 
latter,  we  may  find  one  showing  the  striated  surface  of  triclinic 
feldspar.  We  therefore  conclude  that  both  orthoclase  and  plagio- 
clase  feldspars  are  present  in  this  granite.  The  foliated  structure 
of  the  rock,  together  with  the  finely  granular  condition  of  the  quartz 
and  feldspar,  indicates  that  this  rock  has  undergone  a  severe  crush- 
ing by  which  these  minerals  were  pulverized  and  arranged  with  a 
parallelism  of  position,  giving  a  foliation  and  fissility  to  the  rock. 
This  granite  is  clearly  distinguished  from  the  small,  tourmaline 
granite  masses  seen  at  the  quarry  to  the  north.  Tourmaline  is 
almost  entirely  lacking  in  this  granite.  In  only  one  place  do  we 
find  it,  and  then  on  the  border  of  the  schist,  and  in  small  quantity. 
This  granite  also  contains  two  or  three  micas,  where  that  rarely 
contained  one;  and  this  contains  more  triclinic  feldspar.  As  we 
look  about  us,  we  see  this  granite  extending  quite  a  distance  to 
the  south,  and  so  we  follow  the  outcrops,  that  we  may  represent 
on  the  map  the  area  under  which  the  granite  extends.  Before 
we  are  through  tracing  its  boundaries  by  means  of  the  outcrops, 
we  conclude  that  this  granite  constitutes  quite  a  large  mass,  yet 
somewhat  smaller  than  that  of  Millstone  Hill.  This  granite  ex- 
tends south  to  the  house  of  the  late  A.  Swan  Brown;  to  the  west, 
to  the  top  of  the  hill,  and  a  few  hundred  feet  west  of  the  house 


136  GEOLOGY   OF  WORCESTER. 

there;  and  to  the  north,  under  the  loose  glacial  material,  some 
distance,  though  exactly  how  far  cannot  be  definitely  determined 
from  lack  of  outcrops.  This  granite  may  then  be  represented  on 
the  map  as  an  oval  area,  a  mile  and  one  eighth  in  length  by  about 
a  quarter  of  a  mile  in  width. 

This  is  only  one  of  the  many  granite  areas  found 
Granite  widely     wjthin  this  Paxton  schist,  as  the  schist  is  traced  from 

distributed  in  .      . 

Paxton  schist,  town  to  town,  for  it  is  the  rock  seen  at  the  surface 
in  broad  areas  to  the  west  in  the  plateau  of  Central 
Massachusetts.  But  frequently  the  granite  in  this  schist  is  in 
much  smaller  masses,  at  times  being  but  a  few  feet  in  thickness, 
and  constituting  bands  that  alternate  with  bands  of  the  schist. 
These  granite  bands  tell  us  of  the  rising  from  beneath  of  molten 
rock,  while  the  schist  was  far  beneath  the  surface,  separating  the 
strata  of  the  sedimentary,  or  the  laminae  of  the  recrystallized  rock, 
and  forming  sheet-like  masses  between  them.  In  other  cases  the 
molten  rock  was  intruded  in  larger  bodies  and  widely  separated 
the  strata  or  laminae.  In  these  different  positions  the  molten 
rock  slowly  crystallized,  constituting  the  granite  sheets  and  areas 
now  found  within  this  schist. 

If  we  continue  our  study  to  the  south,  we  shall 

Extension  of 

the  Paxton       find  in  the  outcrops  this  same  mixture  of  Paxton 

schist  to  the      schist  and  granite  extending  under  Cherry  Valley, 

then  appearing  in  a  cut  by  the  side  of  the  railroad 

then  west  of  Jamesville,  and  farther  south  outcropping  frequently 

in  Auburn,  Oxford  and  Dudley,  but  always  lying  to  the  west 

of  the  Carboniferous  quartzite,   which  we  have  studied  in   the 

second   chapter.      But  this   Paxton   schist,  mixed   with    granite, 

does  not  constitute  a  narrow  band  just  west  of  the  Carboniferous 

quartzite;  it  extends  over  a  broad  area,  underlying  many  towns, 

to  the  west,  in  Central  Massachusetts. 

Returning  to  the  quarry  on  the  side  of  Tatnuck 
Extension  of      jjjji  from  which  we  started  in  our  study  to  the  south. 

Paxton  schist 

to  the  north.  let  us  in  like  manner  study  to  the  north.  We  may 
find  the  same  schist  at  the  Cascade.  This  succession 
of  waterfalls  is  produced,  in  part,  by  the  water  of  this  brook  flowing 
over  the  edges  of  the  laminae  of  this  Paxton  schist.  Here  these 
laminae  point  about  north  and  south,  and  have  a  small  dip  of 
twenty-five  degrees  to  the  west.  Thence  we  may  trace  this  schist 
along  the  Holden  Reservoir  road  into  Holden.  and  on  to  the  north, 


GEOLOGY    OF   WORCESTER.  137 

until  it  disappears  under  an  overlying  formation.  Throughout 
this  extent,  the  schist,  or  the  granite  belonging  with  it,  is  adjacent 
to,  and  west  of  the  Carboniferous  quartzite,  the  two  occupying 
the  same  relative  positions  to  the  north  that  they  do  to  the  south. 
Relation  of  We  are  next  led  to  inquire  what  relation  the  Paxton 

Paxton  soiiist     schist   and    Carboniferous    quartzite  really  bear  to 
er°ous  quartz-      each  other.     To  solve  this  problem,  let  us  go  out  on 
ite-  Chandler  street  to  a  point  a  short  distance  west  of 

the  junction  with  Hadwen  lane.     There,  on  the  northeast  side  of 
the  street,  may  be  found  a  large  outcrop  presenting  the  fresh  rock. 
Even  at  a  distance  we  notice  the  laminated  struct- 
Ledge  by  side     ure,  because  the  edges  of  the  laminae  point  directly 
"Leet  "fear       towards  the  street.     They  strike  twelve  degrees  east  of 
Hadwen  lane,      north,  and  dip  fifty- two  degrees  west.     The  rock  is  of 
a  grey  color  with  a  brownish  tint;  is  of  finely  granu- 
lar texture;   consists  largely  of  very  fine,  glassy  quartz,  mingled 
with  considerable  fine  brown  mica,  which  gives  a  brownish  tint 
to  the  whole  rock.     If  we  compare  this  rock  with  that  in  Pleasant 
street,  west  of  Newton  square,  or  with  that  at  the  foot  of  Chad- 
wick  street,  we  shall  instantly  conclude  that  all  these  rocks  are 
one  and  the  same  formation.     This  idea  is  confirmed  by  the  strike 
of  the  laminae  in  the  Chandler  street  ledge,  for  they  are  pointing 
almost  directly  across  to  those  in  Pleasant  street.     We  therefore 
conclude  that  we  are  still  on  the  Carboniferous  quartzite. 
Tourmaline  But  while  we  are  at  the  ledge  by  the  side  of  Chandler 

granite  in        street,  we  may  notice,  in  the  midst  of  the  quartzite, 
^thesideot        a  TOC^>  nearly  white  in  color,  which  does  not  conform 
chandler         to  the  laminae,  but  is  wrapped  around  by  them. 
In  structure  this  white  rock  is  massive,  and  cut  by 
joints  into  irregular,  angular  blocks,  varying  in  size.     In  texture 
it  is  largely  finely  granular,  becoming,  in  places,  quite  coarse. 
We  immediately  recognize  in  it  the  colorless,  glassy  quartz,  and 
the  white,  porcelain-like  feldspar.     The  latter  is,  in  part,  quite 
coarse,  and  has  a  grey  color.    Mica  is  almost  entirely  absent.    There 
are,  however,  fine,  columnar  crystals,  approximately  parallel  in 
position,  distributed  through  this  rock.     These  little  crystals  have 
a  resinous  lustre,  and  a  brownish  yellow,  or  dark  amber,  color. 
They  prove  to  be,  on  testing,  little  crystals  of  tourmaline.     This 
white  rock  is  a  small  intruded  or  injected  mass  of  tourmaline 
granite  in  the  Carboniferous  quartzite.     The  occurrence  of  this 


138  GEOLOGY   OF   WORCESTER. 

tourmaline  granite  here  reminds  us  of  the  tourmaline  granite  found 

in  the  Paxton  schist,  at  the  quarry  near  the  Paxton  road. 

Apatite  in  While  studying  this  granite,  our  attention  is  at- 

tourmaiine        tracted  by  little  deep-green  particles,  distributed,  here 

chandler         and  there,  in  it.     These  particles  are  generally  longer 

street.          than  they  are  thick,  indicating  a  prismatic   shape; 

and  they  have  a  glassy  lustre,  and  are  not  so  hard  but  they  may 

be  scratched  by  the  knife  blade.     On  being  tested,  they  are  found 

to  dissolve  in  nitric  acid,  and  this  solution  gives  a  decided  test 

for  phosphoric  acid.      These  characteristics   clearly  indicate  that 

this  mineral  is  a  deep-green  apatite. 

In  our  study  of  the  rocks  by  the  side  of  Chandler 

Study  of  the  J  * 

ledge  in  Mill      street,   we   have,   perhaps,   forgotten   the  object   in 

8tljuneear  view'  We  wish  to  determine  the  relation  of  the 
Carboniferous  micaceous  quartzite  to  the  Paxton 
schist.  From  Chandler  street  we  will  seek  other  outcrops  nearer 
the  Paxton  schist.  Unfortunately,  the  drumlins  in  the  western 
part  of  Worcester  are  unusually  abundant,  and  securely  conceal 
the  rock-floor  over  large  areas.  We  go  through  June  street  to 
the  west,  because  we  shall  then  be  going  directly  towards  the  Paxton 
schist.  We  do  not  find  an  outcrop  until  we  reach  Mill  street. 
Just  south  of  the  junction  of  these  two  streets  is  a  large  ledge 
appearing  by  the  side  of  the  street,  and  presenting  the  freshly 
broken  rock  for  our  study.  Other  outcrops  may  be  found  in  the 
neighboring  fields,  on  the  west  of  Mill  street. 

The  rock  of  these  ledges  is  of  a  dirty  grey  color 

of  the  rock  on  weathered  surfaces,  and  thinly  laminated  in 
ofbMine street  structure-  Tne  laminae  strike  eighteen  degrees  east 
of  north,  and  dip  thirty-seven  degrees  west.  The 
rock  is  dense  and  hard,  and,  under  the  hammer,  breaks  with  a 
roughly  conchoidal  fracture.  It  is  also  somewhat  tough  and  quite 
brittle.  On  the  freshly  broken  surface,  the  rock  is  seen  to  be 
finely  granular  in  texture,  and  of  a  brownish  grey  color.  This  rock 
is  a  little  more  decidedly  brownish  than  was  the  Chandler  street 
rock.  This  Mill  street  rock  consists  largely  of  finely  granular 
quartz,  mixed  with  a  fine,  brown  mica.  The  mica  is  more  abundant 
than  it  was  in  the  Chandler  street  rock,  hence  the  darker  color; 
but  is  not  more  abundant  than  is  the  mica  in  much  of  the  Carbon- 
iferous quartzite.  The  latter  rock,  as  it  appeared  in  East  Kendall 
street,  was  quite  as  micaceous.  The  Mill  street  rock  is  also  a 


GEOLOGY    OF   WORCESTER.  139 

little  coarser  ifi  texture  than  the  Chandler  street  rock.  In  ad- 
dition to  the  quartz  and  mica,  the  Mill  street  rock  also  contains 
disconnected,  small  masses,  or  irregular  veins,  of  granite,  more 
or  less  flattened,  or  lengthened,  in  the  direction  of  the  laminae. 
This  granite  frequently  contains  black  tourmaline.  Associated 
with  the  granite,  though  not  really  belonging  to  it,  is  a  coarse, 
dark-green  hornblende,  in  small  bladed  masses.  These  latter 
characteristics  are  identical  with  those  of  the  rock  in  Tatnuck 
Hill,  especially  with  those  of  the  rock  in  the  quarry  near  the  Pax- 
ton  road. 

In  texture  this  Mill  street  rock  is  finer  than  the  Paxton  schist. 
In  other  words,  the  rock  we  are  now  studying  has  characteristics 
connecting  it  with  the  Paxton  schist  to  the  west,  and  other  charac- 
teristics connecting  it  with  the  Carboniferous  micaceous  quartz- 
ite  on  the  east.  It  is  the  transition  link  between  the  two. 

The  outcome  of  this  study  is  that  the  Paxton 
Paxton  schist     schist  is  only  a  coarser  variety  of  the  Carboniferous 

a  coarser 

phase  of  the      quartzite,  coarser  because  more   highly  metamorph- 
carboniferous      Osed,  and   more   thoroughly  recrystallized.     But  we 

micaceous 

quartzite.  may  reasonably  ask  why  one  has  thus  been  more 
thoroughly  recrystallized  and  metamorphosed.  We 
have  noticed,  as  we  worked  towards  the  west  in  tracing  this  con- 
nection, that  there  was  an  increasing  quantity  of  injected  granite 
in  the  schist.  Near  by,  in  the  top  of  Tatnuck  Hill,  is  a  large  area 
of  granite.  To  the  west  in  the  plateau,  wherever  the  Paxton  schist 
is  found,  it  contains  more  or  less  injected  granite.  To  this  injected 
granite,  and  to  the  probably  greater  masses  buried  beneath,  from 
which  these  dikes  at  the  surface  are  offshoots,  we  may  attribute 
the  greater  metamorphism  in  the  Paxton  schist.  A  like  effect 
was  produced  by  the  granite  of  Millstone  Hill  in  the  Carboniferous 
micaceous  quartzite  in  East  Kendall  street. 

But  we  have  already  traced  a  like  transition  from 

Paxton   schist 

and  Boiton       the  Carboniferous  micaceous  quartzite,  on  the  east, 

ene1enetsUiVa"  into  Bolton  Sneiss-  The  Paxton  schist  and  the 
Boiton  gneiss  are,  then,  equivalents  or  simply  parts 
of  one  extensive  formation.  Studying  these  two,  separated  by 
many  miles,  the  geologist  can  not  but  note  the  many  resemblances 
between  them.  The  Paxton  schist  has  been  described  as  a  quartz- 
ose  mica  schist,  of  a  brownish  grey  color,  and  frequently  contain- 
ing alternating  lighter  bands  of  a  greenish  color.  In  the  quarry 


140  GEOLOGY   OF   WORCESTER. 

at  Quinsigamond1  the  schist  is,  in  large  part,  made  up  of  alternat- 
ing bands  of  brown  and  light  green.  So  closely  do  specimens 
from  the  Quinsigamond  and  Tatnuck  Hill  quarries  resemble  each 
other,  that  they  might  reasonably  be  considered  as  coming  from 
the  same  quarry.  The  resemblance  extends  even  to  the  light 
green  bands  in  each,  containing  small,  light  amber  colored  garnets. 
The  Paxton  schist  and  Bolton  gneiss  also  resemble  each  other  in 
containing  a  vast  quantity  of  parallelly  injected  granite.  Some- 
times this  granite  appears  in  dikes  a  few  inches  thick,  between 
the  laminae;  sometimes  it  is  in  the  form  of  batholites  a  mile  or 
more  in  width.  The  granite  in  one  formation  generally  resembles 
that  in  the  other.  Each  is  a  coarse  granite  containing  much 
coarse,  white  feldspar,  which  occurs  in  large,  rounded,  flattened, 
cleavable  individuals,  rather  than  in  distinct,  porphyritic  crystals. 
These  granites  are  generally  foliated  parallelly  to  the  lamination 
of  the  schist,  giving  a  marked  gneissoid  structure.  When  the 
injected  granite  does  not  form  a  continuous  dike, 

Pseudo-con-  , J 

glomerate        the  coarser  mica  laminae  wrap  around  the  flattened 

of  Paxton  feldspars  producing  a  pseudo-conglomerate.  The 
pseudo-conglomerate  is  quite  as  characteristic  of  the 
Paxton  schist,  as  it  is  of  the  Bolton  gneiss,  and  may  be  seen  in 
perfection  in  outcrops  near  the  dam  of  the  Holden  Reservoir,  and 
in  the  large  ledges  near  the  Charlton  station  of  the  Boston  and 
Albany  railroad. 

Difference  But  while  there  are  these  marked  resemblances 

between  Pax-  between  the  Bolton  gneiss  and  Paxton  schist,  there 

and  Boiton  is  one  marked  difference.  The  Bolton  gneiss  con- 
gneiss,  tains  many  bands  of  a  coarse,  black,  thinly  laminated 
mica  schist,  more  or  less  chloritic.  This  mica  schist  is  rarely,  if 
ever,  found  in  the  Paxton  schist.  While  the  Paxton  schist  was 
made  out  of  layers  of  impure  sand  by  a  partial  recrystallization, 
the  Bolton  gneiss  was  in  part  made  from  such  layers,  and  in  part 
from  layers  that  were  more  nearly  pure  clay.  This  difference 
simply  indicates  that  the  original  sediments  to  the  west  were  more 
sandy,  and  to  the  east,  more  clayey.  The  more  clayey  sediments 
indicate  more  quiet  and  deeper  waters,  in  which  the  sediments 
were  laid  down. 

Another  difference  between  the  Paxton  schist  and  the  Bolton 
gneiss  is  that  there  are  no  limestone  areas  in  the  former  like  those 
in  the  latter.  This,  however,  is  a  difference  of  minor  importance, 

1  Ballard  quarry. 


PSEUDO-METAMORPHIC    CONGLOMERATE,    FROM    NEAR    CHARLTON     STATION 

OF  B.  &  A.  R.  R.    FROM  THE  PAXTON  SCHIST. 


GEOLOGY   OF    WORCESTER.  141 

for  the  limestone  in  the  Bolton  gneiss  is,  perhaps,  only  accidental. 
It  is  possibly  limestone  that  was  deposited  from  calcareous  springs 
rather  than  limestone  formed  from  organic  remains.  There  is  in 
these  differences  nothing  to  prevent  our  thinking  of  these  two 
rocks  as  equivalents  or  parts  of  one  extensive  formation. 

BRIMFIELD  SCHIST. 

Before  leaving  the  extreme  western  part  of  Worcester,  there  is 
still  another  formation  demanding  our  attention.  If,  on  leaving 
the  quarry  near  the  Paxton  road,  we  go  west,  instead  of  south  or 
north,  we  shall  soon  find  evidence  that  we  are  on  quite  a  different 
rock.  This  rock  may  be  best  studied  in  the  large  ledges  about 
an  eighth  of  a  mile  west  of  the  quarry,  and  back  of  the  barn  be- 
longing to  Homer  King.  At  the  eastern  foot  of  the  hill,  made  up 
of  this  new  rock,  will  be  seen  the  Paxton  schist,  striking  seven  degrees 
east  of  north,  and  dipping  twenty-five  degrees  west.  The  laminae 
of  this  schist  slope  down  into  the  base  of  this  hill  beneath  the 
new  rock. 

Let  us  now  study  and  describe  this,  the  last  of  the  rocks  con- 
stituting any  considerable  part  of  the  rock-floor  of  Worcester. 
While  this  rock  underlies  only  a  few  square  miles  in  Worcester,  it 
makes  up  the  whole  of  the  rock-floor  under  some  of  the  neighbor- 
ing towns  to  the  west  and  northwest.  It  is  an  extensive  forma- 
tion, though  not  so  important  in  Worcester. 

Brimfleid  ^ne  ^rs^  characteristic  of  this  new  rock  is  its  ex- 

schist  tremely  rusty  color.  It  is  everywhere  charged  with 
iron  rust.  The  predominating  mineral  which  gives 
special  character  to  the  rock  is  mica,  generally  reddish  in  color,  but 
in  places  white  and  silvery.  While  the  mica  is  quite  generally  in 
flat  scales,  as  wide  as  long,  sometimes  these  scales  have  the  appear- 
ance of  having  been  drawn  out,  or  lengthened,  in  the  direction  of  the 
strike,  so  as  to  present  an  almost  fibrous  form.  Because  of  the 
parallelism  in  the  arrangement  of  the  mica  scales  the  rock  has  a 
marked  lamination,  and  splits  along  planes  parallel  to  the  scales. 
Frequently  also  the  laminae  of  the  rock  have  been  crumpled,  devel- 
oping more  or  less  secondary  cleavage,  so  that  the  splitting  of  the 
rock  is  in  places  quite  irregular.  In  addition  to  the  mica,  this 
rock  contains  considerable  finely  granular  quartz,  partly  rusty, 
and  partly  white  and  glassy.  Mixed  with  the  quartz  is  some 


142  GEOLOGY   OF   WORCESTER. 

feldspar,  also  granular  and  rusty  or  white.  These  are  the  essen- 
tial minerals  of  this  rock.  Because  the  mica  predominates  and 
gives  special  character  to  the  rock,  the  rock  is  rightly  called  a  mica 
schist.  But  as  the  extreme  rustiness  is  also  characteristic,  it  may 
be  called  a  rusty  mica  schist.  Because  of  the  occurrence  of  this 
rusty  schist  in  the  rock-floor  of  the  town  of  Brimfield  it  has  been 
called  the  Brimfield  schist. 

Minerals  in  Before  considering  this  rusty  schist  in  its  wider 

Brimfield        relations,  let  us  examine  it  more  closely  for  other 

minerals.  At  first  we  find  very  little  to  reward  our 
search,  but  after  a  time  we  may  find  a  mineral  occurring  in  white, 
or  slightly  rusty,  small  masses  or  clumps,  made  up  of  fine  glassy 
fibres.  It  is  the  mineral  fibrolite  or  sillimanite,  which  we  have 
already  described  as  occurring  in  a  mica  schist  at  the  red  bridge 
over  the  B.  &  A.  R.  R.,  near  Bloomingdale,  and  in  the  rusty  schist 
that  was  associated  with  the  Shrewsbury  diorite.  While  searching 
for  the  fibrolite,  we  may  possibly  notice  fine  scales  of  a  dark  grey 
color,  shining  with  a  metallic  lustre.  They  are  scales  of  graphite. 
In  the  midst  of  this  rusty  area  is  a  mass  of  coarse  tourmaline 
granite;  in  the  schist  bordering  this  granite  may  be  found  little 
black  prisms  of  tourmaline.  These  have  been  developed,  probably, 
in  the  schist  by  its  contact  with  the  granite.  These  are  the  min- 
erals noticed  in  this  rusty  mica  schist  at  this  place,  though  gar- 
nets are  frequently  found  in  it  elsewhere. 

Extension  of  We  may  nex^  inquire  as  to  the  extent  of  this  rusty 
the  Brimfield  schist.  We  follow  this  ledge  south  through  this 

field  and  into  the  next.  The  loose  glacial  material 
covers  the  ledges  until  we  have  crossed  Fowler  street.  As  we 
have  already  pointed  out,  only  the  Paxton  schist  and  granite  ap- 
pear south  of  this  street;  then,  evidently,  this  rusty  schist  does 
not  extend  far  south  of  where  we  see  it  in  this  ledge  already  stud- 
ied. To  the  north,  however,  we  may  find  it  at  the  Cascade  where 
the  uppermost  laminae,  over  which  the  water  falls,  are  of  this  rusty 
schist.  Thence  we  may  accurately  trace,  by  means  of  the  numer- 
ous outcrops,  the  boundary  line  between  the  rusty  Brimfield  and 
the  Paxton  schists  into  Holden.  This  line  is  found  west  of  the 
road1  extending  from  Tatnuck  northwesterly  by  the  side  of  the 
Holden  Reservoir.  This  boundary  is  indicated  on  the  geological 
map.  West  of  that  line,  for  many  miles,  the  surface  rock  is  the 
rusty  mica  schist,  containing  larger  or  smaller  masses  of  intruded 

1  Olean  street. 


GEOLOGY   OF   WORCESTER.  143 

granite.  Such  is  the  rock  of  Asnebumskit,  rising  fourteen  hun- 
dred feet  above  the  sea,  and  four  hundred  feet  above  the 
surrounding  plateau. 

Having  studied  this  rusty  Brimfield  schist  as  to 
Brimfield  amf  minerals  contained  and  as  to  extent,  we  may  inquire 
Paxton  schists  what  relation  it  bears  to  the  Paxton  schist,  to  which 


^  adjacent  In  the  hill  where  we  studied  the 
rusty  schist,  we  saw  the  Paxton  schist  at  the  eastern 
foot,  striking  seven  degrees  east  of  north  and  dipping  twenty- 
five  degrees  west.  In  other  words  the  Paxton  schist  dips,  or  slants, 
down  beneath  the  rusty  schist,  which  is  in  the  upper  part  of  the 
hill.  At  first  thought  we  might  conclude  that  this  settles,  once 
for  all,  the  relation  of  these  two  rocks,  and  that  the  Brimfield  schist 
must,  of  necessity,  be  above,  or  rest  on,  the  Paxton  schist.  But 
it  must  be  borne  in  mind  that  we  are  in  a  region  where  the  rock 
strata  have  been  folded  to  the  superlative  degree,  and  the  rock  sub- 
stance frequently  rearranged,  producing  secondary,  if  not  even 
tertiary,  structure,  thus  hiding  the  original  structure  or  bedding. 
There  are  large  and  small  folds.  There  are  anticlinal,  or  arch- 
shaped  folds,  and  synclinal,  or  trough-shaped  folds.  The  folds  may 
be  open,  their  sides  spread  apart,  or  they  may  be  compressed  and 
overturned  —  their  sides  pressed  together  so  that  the  laminae  in 
one  side  are  parallel  with  those  in  the  other,  and  then  the  whole  fold 
tipped  over  so  that  all  the  laminae  slope  in  one  direction  except 
at  the  very  top  or  bottom  of  the  fold.  Where  such  severe  fold- 
ing has  taken  place,  it  does  not  necessarily  follow  that,  because  the 
Paxton  schist  dips  under  the  rusty  Brimfield  schist  in  one  place, 
the  former  was  really  beneath  the  latter  in  their  original  posi- 
tions.1 These  relative  positions  may  have  resulted  from  the 
overturning  of  a  fold,  thus  causing  that  which  was  beneath  to  be 
above  and  resting  on  the  originally  upper  one.  If  we  could  trace 
out  the  whole  of  a  fold  here,  we  might  determine  the  relation  of 
these  two  rocks  ;  but  this  we  are  not  able  to  do,  partly  because 
of  the  limited  area  of  the  exposed  rock,  and  partly  because  of  the 
confusion  of  structures  within  the  rusty  schist. 

We  therefore  seek  another  locality  where   these 
rocks  may  be  seen  near  to  each  other.      They  appear 
aSam  together  at  the  Cascade.     This  is  situated  west 
at  the  cascade,     of  Olean  and  Cataract  streets  and  about  one  third  of 
a  mile  north  of  Liberty  Farm.     Here  a  small  brook 

1  See  the  discussion  in  regard  to  rocks  in  Ballard  field,  pp.  124,  125,  126,  127. 


144  GEOLOGY   OF   WORCESTER. 

flows  down  the  lower,  precipitous  edge  of  the  plateau,  consti- 
tuting a  series  of  small  water  falls,  called  the  Cascade.  The 
precipitous  slope  consists  of  the  projecting  edges  of  the  laminae 
of  the  rocks  of  the  plateau. 

Commencing,  in  our  study,  at  the  foot  of  the  Cascade,  we  find 
the  Paxton  schist  in  its  well  defined,  slab-like  laminae,  striking 
about  north,  and  dipping  twenty-five  degrees  west.  As  we  climb 
the  steep  slope,  constantly  examining  the  rock  of  the  laminae, 
we  find  only  the  Paxton  schist,  until  we  have  ascended  two  thirds, 
or  three  quarters,  of  the  way,  when  we  begin  to  find  an  alternation 
of  the  Paxton  and  rusty  Brimfield  schists.  Then  the  proportion 
of  the  latter  increases  until,  at  the  top  of  the  slope,  the  rock  is  prac- 
tically all  rusty  Brimfield  schist.  Again  the  Paxton  schist  is  dip- 
ping beneath  the  Brimfield  schist,  and  seems  to  blend  into  it,  with 
no  well  defined  line  between.  This  blending  or  alternation  may 
mean  a  change  from  sandy  layers  of  deposition  to  clayey  layers, 
when  these  beds  were  deposited  in  the  waters  of  an  ancient  sea;  or, 
possibly,  this  alternation  may  result  from  a  minute  interfolding 
or  intercrumpling  of  the  laminae  of  these  two  rocks  along  the  bor- 
der between  them.  If  the  former  of  these  ideas  is  true,  then  clear- 
ly the  Paxton  is  now  beneath  the  Brimfield  schist  and  possibly 
represents  the  older  sediments,  and  is  the  older  formation.  But  if 
the  second  idea  is  the  correct  one,  then  the  relative  positions  mean 
nothing,  and  do  not  determine  for  us  the  actual  relation  of  these 
two  rocks.  Which  of  these  ideas  may  really  represent  the  truth, 
we  have  not  found  possible  to  determine  at  this  locality.  Again 
we  are  left  in  doubt  in  regard  to  the  absolute  relation  of  these 
two  rocks. 

And  so  we  may  follow  along  to  the  north  on  this  boundary  be- 
tween the  Paxton  and  Brimfield  schists,  which  is  a  crooked  line 
lying  west  of  Olean  street,  without  solving  this  question,  because 
nowhere  does  enough  of  a  fold  show  to  make  the  true  relation 
of  these  two  formations  clear. 

Remnants  ^n  an  area  wnere  profound  erosion  has  taken  place, 

of  Brimfleid  as  there  has  here  in  Central  Massachusetts,  it  some- 
times happens  that  mere  remnants  of  a  formation 
have  been  left  as  detached  areas.  Such  remnants,  being  small 
in  extent  and  on  a  small  scale,  sometimes  clearly  present  facts  that 
may  be  concealed  and  overlooked  in  broader  areas.  Let  us  seek 
out  one  of  these  remnants  in  this  study. 


THE  CASCADE,  ONE-THIRD  MILE  NORTH  OF  LIBERTY  FARM. 


GEOLOGY    OF   WORCESTER.  145 

Remnant  We  must  return  to  Tatnuck,   and  follow  North 

in  hillside        Bend  street  to  its  northern  extremity;    up  on  the 
North  Bend       high  side  hill  to  the  northeast  we  see  many  ledges, 
street.  ^s  we  ascend  the  slope,  we  at  first  find  ledges  which 

we  unhesitatingly  identify  as  belonging  to  the  Paxton  schist.  To 
find  the  Paxton  schist  here  is  what  we  might  rightly  expect,  for 
all  along  Olean  street,  on  the  western  side  of  Tatnuck  brook,  we 
find  this  same  schist  except  at  the  northern  end  of  Cataract  street, 
where  we  find  the  Brimfield  schist  extending  as  far  east  as  Olean 
street.  But  on  going  a  little  higher  on  this  side  hill,  and  but  a  few 
hundred  feet  to  the  north,  we  observe  a  sudden  change  in  the  ap- 
pearance of  the  rock.  The  rock  is  now  a  rusty,  thinly  fissile,  mus- 
covitic  and  biotitic  schist  badly  crumpled  into  fine  folds,  and  con- 
taining many  wavy  quartz  veins  and  considerable  included  granite. 
In  general  outward  appearance  this  rock  closely  resembles  the  Brim- 
field  schist,  and  leads  us  to  look  for  the  two  minerals,  graphite 
and  fibrolite,  so  characteristic  of  that  schist.  We  may  be  obliged 
to  search  some  time  for  those  minerals  for  they  are  not  everywhere 
abundant  here;  but  by  persevering,  the  graphite  may  be  found 
here,  and,  a  little  farther  to  the  north,  this  same  rusty  schist  is  high- 
ly fibrolitic.  This  schist  has  all  the  marks  of  the  Brimfield  schist, 
though  it  is  not,  as  far  as  we  can  see  from  this  point,  directly  con- 
nected with  the  Brimfield  schist  on  the  other  side  of  Tatnuck 
brook,  a  mile  or  two  distant.  Moreover  on  studying  the  ledges 
in  an  easterly  direction  we  find  that  this  rusty  schist  is  of  small  ex- 
tent in  that  direction,  appearing  only  in  the  next  field,  with  the 
Paxton  schist  east  of  it.  The  Brimfield  schist  is  here  only  a  few 
hundred  yards  wide,  for  it  is  made  up  of  laminae  pointing  north- 
erly; and  the  rock-floor  is  well  exposed  in  broad  areas  showing 
the  Paxton  and  Brimfield  schists  near  to  each  other, 
position  of  This  'ls>  then,  an  excellent  place  in  which  to  study 

schists  in         the  relation  of  these  two  rocks.     We  therefore  care- 
lide'      fully  take  the  dip  and  strike  of  the  laminae  of  both 
schists  in  different  parts  of  this  hill.     In  the  western  part  of  the 
Brimfield  schist  the  laminae  strike  20-25°  east  of  north  and  dip 
northwesterly;  in  the  extreme  eastern  part  of  the  same  schist  the 
laminae  strike  northwesterly  and  dip  northeasterly.     The  Paxton 
schist  is  found  in  this  hill,  west  and  south  and  east  of  the  Brim- 
field  schist;  in  the  western  part,  striking  northeasterly  and  dipping 
northwest ;  in  the  southern  part  striking  east  and  west  and  dipping 
11 


146  GEOLOGY   OF   WORCESTER. 

north  ten  to  fifteen  degrees;    and  in  the  eastern  part,  striking 

northwesterly  and  dipping  northeasterly. 

illustration  Putting  these  observations  together,  the  meaning 

to  explain        thereof  is  apparent.     Place  a  book,  having  a  paper 

the  position  ' 

of  these         cover,  flat  on  the  table  and  lengthwise  north  and 

schists.  south;  raise  the  southern  end  of  the  book  so  that 
the  upper  surface  of  the  book  is  an  inclined  plane  sloping  fifteen 
degrees  to  the  north;  press  the  east  and  west  edges  of  the  book 
towards  each  other,  bending  the  leaves  up  into  a  fold  with  the 
convexity  up;  now  while  you  hold  the  book  thus  bent,  let  some 
one,  with  a  sharp  knife,  cut  along  a  horizontal  plane  through  the 
leaves,  thus  removing  the  part  of  the  book  above  this  plane.  If, 
now,  we  look  at  the  edges  of  the  leaves  in  this  horizontal  plane 
(what  remains  of  the  book  being  bent  as  at  first )  we  see  the  leaves 
in  the  western  part  of  the  folded  book  pointing  or  striking  north- 
easterly and  dipping  or  slanting  down  northwesterly ;  in  the  eastern 
part,  the  leaves  striking  northwesterly  and  dipping  northeasterly; 
and  along  the  central  line,  extending  north  and  south  through  the 
plane  produced  by  the  cutting,  the  leaves  striking  east  and  west 
and  dipping  north. 

Relative  posi-  ^6  observations  on  the  position  of  the  leaves  of 
tions  of  these  this  book  tally  exactly  with  the  observations  on  the 
two  schists.  iamjnae  Of  the  two  schists  in  this  side  hill.  The 
position  of  the  leaves  exactly  represents  the  position  of  the  laminae. 
The  laminae  of  these  schists  are,  then,  bent  into  an  anticlinal  (con- 
vexity upward)  fold,  whose  axis  or  topmost  line  is  not  horizontal, 
but  inclined  ten  to  fifteen  degrees  to  the  north;  and  this  fold  has 
been  in  part  cut  away  by  the  agents  of  erosion  during  geologic 
ages.  This  fold  is  not  overturned  or  compressed;  it  is  an  open 
anticlinal  fold.  The  rock  that  is  beneath  in  this  fold  is  the  one 
that  was  originally  beneath.  We  are  thus  able  to  determine  the 
original  relative  positions  of  these  two  schists  at  this  place,  be- 
cause we  see  the  top  together  with  a  little  of  each  side  of  this  fold. 

In  the  southern  part  of  this  hill  the  Paxton  schist  strikes  east 
and  west  and  dips  north  down  under  the  Brimfield  schist.  The 
former  makes  up  the  lower  part  of  this  fold,  and  hence  is  the  lower 
of  these  two  rocks. 

On  the  opposite,  or  western,  side  of  Tatnuck  brook,  these  schists 
strike  between  north  and  northeast  and  dip  northwesterly  and  are 
in  the  western  side  of  this  fold,  showing  that  this  fold,  of  which 


GEOLOGY   OF   WORCESTER.  147 

we  now  see  a  small  remnant,  was  a  broad  and  extensive  fold. 
Tatnuck  brook  valley  has  been  cut  in  the  western  side  of  this 
fold,  and  has  been  cut  through  the  Brimfield  schist  down  into  the 
Paxton  schist. 

This  last  fact  shows  us  that  in  this  area  the  Brim- 

Brimfieid        field  schist  is  thin  and  is  measured  in  depth  by  only 

schist  in  this      a  few  feet  or  few  hundred  feet  at  the  most.     This, 

side  hill.  .  .    .  . 

however,  gives  us  no  idea  of  the  original  thickness 
of  this  formation  or  of  its  thickness  in  other  places,  because  in  this 
anticlinal  the  Brimfield  schist  grows  thinner  and  thinner  as  the 
folded  laminae  slant  up  to  the  south. 

small  anti-  We  nave  now  solved  the  problem  that  led  us  to 

ciines  in  this  this  side  hill,  but  there  are  other  facts  here  worthy 
side  hiii.  of  notice  In  the  secon(i  fi^d,  to  the  east,  is  a  large 
outcrop  of  Brimfield  schist,  whose  southern  end  is  marked  by  two 
large  white  pine  trees.  In  this  outcrop,  within  a  width  of  one 
hundred  feet  or  less,  may  be  traced  the  laminae  striking  north- 
easterly and  dipping  northwesterly,  striking  east  and  west  and 
dipping  northerly,  finally,  in  the  eastern  part  of  the  ledge,  striking 
northwesterly  and  dipping  northeasterly.  Again  about  three  hun- 
dred feet  northwest  of  this,  there  may  be  traced  another  small 
anticlinal  fold  also  pitching  or  sloping  to  the  north.  These  are 
small  folds  produced  during  the  formation  of  the  large  fold,  by 
the  crumpling  of  the  rock  laminae  near  the  axis  or  central  line 
of  the  larger  fold. 

Tourmaline  In  ^otl1  of  tne  schists  of  this  locality  we  find  much 

granite  in        intruded  granite,  coarse  in  the  central  part,  and  fre- 

these  schists.      quently   fine   near   tne   border   of   the   mass.     This 

granite  is  white  but  contains  black  tourmaline,  frequently  in  large 
prisms  in  the  coarse  granite,  but  in  fine  ones  near  the  contact  with 
the  schist.  In  places  these  little  tourmalines  are  so  abundant  as 
to  cause  the  granite  to  be  black.  From  such  masses  of  tourmaline 
granite  have  'come  the  many  bowlders  of  tourmaline  granite  found 
in  the  more  southern  part  of  Worcester. 

But  there  is  a  still  more  interesting  fact  brought 
FTide  hiiihl8  out  clearlY  in  this  side  hill.  Probably  in  our  study, 
in  this  field,  of  the  relation  of  these  two  schists,  we 
have  noticed  that  the  Paxton  schist,  found  to  the  east  of  the  Brim- 
field,  while  striking  northwesterly  and  dipping  northeasterly,  is  at 
a  considerably  higher  level  in  the  hill  than  is  the  Brimfield.  If, 


148  GEOLOGY   OF   WORCESTER. 

also,  we  follow  this  eastern  line  of  Paxton  schist  by  outcrops  to 
the  northwest  along  the  line  of  strike,  for  a  quarter  of  a  mile  or 
so,  we  may  also  find  the  Brimfield,  just  west  and  dipping  north- 
easterly, apparently  sloping  directly  beneath  the  Paxton  schist. 
This  observation  does  not  at  all  invalidate  the  conclusion  as  to 
which  rock  is  really  the  lower  one.  The  anticline  is  too  clear  and 
convincing.  This  apparent  dipping  of  the  upper  beneath  the 
lower  schist  indicates  that  more  than  simple  folding  has  here 
taken  place.  The  rocks  have  been  broken,  and  the  line  of  break- 
ing had  a  northwest-southeast  direction;  the  rocks  on  the  north- 
east side  of  this  line  were  raised  or  elevated  so  as  to  bring  a  part 
of  the  Paxton  up  on  a  level  with,  and  even  to  a  higher  level  than, 
a  part  of  the  Brimfield  which  is  on  the  southwest  side  of  this  fract- 
ure. This  observation  serves  to  reveal  to  us  a  fault  in  the  eastern 
side  of  this  large  fold. 

This  fault  is  probably  not  the  only  one  in  this  anticline,  for  as 
we  follow  and  study  this  narrow  area  of  Brimfield  schist  to  the 
north,  there  is  evidence  that  this  schist  does  not  thicken  as  rapidly 
as  might  be  expected  from  the  northern  slope  or  pitch  of  this  anti- 
clinal fold.  A  mile  or  so  north,  the  Brimfield  is  evidently  thin  for 
the  Paxton  may  be  found  in  places  in  its  midst  as  if  the  lower  schist 
had  been  exposed  by  the  wearing  off  of  the  upper.  This  failure 
in  the  Brimfield  schist  to  thicken  with  the  pitch  of  the  anticline 
may  be  explained  by  supposing  other  faults  crossing  this  anticline, 
and  thus  bringing  the  Paxton  schist  nearer  to  the  surface  to  be 
exposed  more  readily  by  erosion.  The  meaning  of  this  is  that 
this  anticlinal  fold,  instead  of  being  made  up  of  continuous  rock 
laminae,  has  been  broken  into  a  succession  of  blocks  of  rock,  and 
each  block  has  been  moved  up  a  little,  or  elevated,  with  reference 
to  the  next  block  to  the  south,  and  then  the  whole  surface  worn 
down  by  subsequent  erosion  nearly  to,  or  in  places  fully  to,  the 
boundary  plane  between  the  Paxton  and  Brimfield  schists,  thus 
sometimes  exposing  the  former  in  the  midst  of  the  latter. 

A  e  of  Before  leaving  this  interesting  area  we  must  notice 

Brimfield         that  this  anticline  has  shown  to  us  more  than  simply 

the  relative  positions  of  these  two  schists.     We  have 

already  identified  the  Paxton  schist  as  a  more  metamorphosed,  or 

highly  crystallized,  phase  of  the  Carboniferous  quartzite,  and  is 

therefore  Carboniferous  in  geologic  age.     The  Brimfield  schist  lies 

above  the  Paxton  schist  just  as  the  Worcester  phyllite  lies  above 


GEOLOGY   OF   WORCESTER.  149 

the  Carboniferous  quartzite;  then  the  Brimfield  schist,  made  from 
clayey  sediments,  corresponds  to,  or  is  a  more  highly  metamorph- 
osed phase  of,  the  Worcester  phyllite,  and  is  therefore  Carbon- 
iferous in  age. 

Pax  ton  and  We  nave  t^lus  far  written  of  tne  Paxton  and  Brim- 

uriinfieid        field  schists  as  local  rocks;  they  are  much  more  than 

rockfTof'the      this.     But  a  small  Pai>t  of  these  formations  is  con- 

i.iateau  of        tained,  as  has  been  intimated,  within  the  bounds  of 

Massacim-        Worcester.     These   two   rocks,   with   their  included 

setts.  granite,  constitute  nearly  the  whole  of  the  plateau 

of   Central    Massachusetts,    from   Worcester   to    the   Connecticut 

valley.     Over  this  broad  area  the  distribution  of  the  Brimfield 

schist  is  somewhat  irregular.     While  the  great  mass  of  it  occurs 

in  extensive  areas,  overlying  the  Paxton  schist,  there  are    many 

small  areas,  like  that  already  studied,  frequently  constituting  less 

than  a  square  mile  of  surface,  entirely  isolated  and  surrounded 

by  the  Paxton  schist.     They,  too,  are  probably  remnants  of  this 

formation.     They  tell  us  of  a  much  larger  former  extension  of 

this  Brimfield  schist.     It  very  likely  covered  the  Paxton  schist 

throughout  the  latter  's  extent,  but  has  been  removed  over  broad 

areas,  exposing  the  underlying  schist,  by  the  profound  erosion, 

by  which   the  surface  of   the  plateau  of  Central  Massachusetts 

was  produced. 

In  these  scattered  areas  of  rusty  Brimfield  schist, 
surrounded  by  the  Paxton,  we  find  something  close- 
ly  resembling  what  we  also  find  east  of  Worcester 
gneiss.     in   the   Bolton  gneiss.      In   the  latter  are  isolated 


areas  of  a  rusty,  fibrolitic,  graphitic,  mica  schist 
identical  in  appearance  with  the  Brimfield  schist.  We  have 
already  noticed  one  such  area  bordering  the  Shrewsbury  diorite 
dike;  we  have  studied  another  area,  east  of  Providence  street, 
which  shows  a  transition  of  the  Carboniferous  phyllite  into  this 
rusty  schist,  and  which  also  shows  its  relation  to  the  Carboniferous 
quartzite,  that  of  an  overlying,  conformable  formation.  Numer- 
ous other  areas  of  this  rusty  schist  might  be  pointed  out  in  the 
neighboring  towns  through  which  the  Bolton  gneiss  extends.  All 
these  confirm  the  belief  that  the  Brimfield  schist  is  above  the  Paxton 
in  the  plateau  west  of  Worcester,  and  is  the  equivalent  of  the  Car- 
boniferous phyllite. 


150  GEOLOGY   OF  WORCESTER. 

Graphite  With  the  Carboniferous  age  of  these  rocks  of  Cen- 

at  tral  Massachusetts  so  well  established,  the  old  graph- 
j^  mine  at  Sturbridge,  which  has  been  sporadically 
worked  for  so  many  years,  comes  into  its  proper  place.  Like  the 
graphite  deposit  here  in  Worcester,  that  deposit  is  of  the  Carbon- 
iferous, and  represents  an  ancient  vegetable  deposit,  now  entirely 
recrystallized  into  graphite.  In  like  manner  the  graphite,  which 
is  found  in  fine  scales  almost  everywhere  in  the  Brimfield  schist, 
also  represents  organic  matter  buried  in  these  clayey  beds,  when 
they  were  deposited  in  that  distant  geologic  time. 


LOOKING  ACROSS  THE  PLATEAU  OF  CENTRAL  MASSACHUSETTS. 


CHAPTER  VIII. 

GENERAL    GEOLOOY   OF  WORCESTER    AND   OF  THE    PLATEAU   OF 
CENTRAL  MASSACHUSETTS. 

A  SUMMARY. 

The    eastern   border    of   the   plateau   of  Central 
central          Massachusetts  is  not  clearly  defined  because  of  the 
Massachu-       long,  gentle  slope  by  which  the  highlands  of  the 
interior  blend  into  the  lowlands  bordering  the  sea; 
but  if  the  500  foot  level  be  assumed  as  the  dividing  line,  we  may 
trace  an   approximate   border  beginning  in   the  south  with   the 
Blackstone  river  valley,  thence  northerly  through  the  towns  of 
Upton,  Westboro  and   Northboro,   crossing  the  head  waters  of 
the  Assabet  river  near  their  sources,  thence  through  Clinton,  across 
the  upper  course  of  the  Nashua  river,  and  through  Leominster, 
Lunenburg  and  Townsend.     Westerly  from  this  irregular  line  the 
plateau  extends  at  an  almost  uniform  level  of  1000  or  1100  feet 
above  sea  level  to  the  Wilbraham  mountains  and  Pelham  hills, 
which  are  the  escarpment  by  which  the  land  surface  descends 
from  the  plateau  to  the  broad  lowlands  of  the  Connecticut  valley. 
This   plateau   is   trenched   easterly   by   the   head 
the'irtatea'u       waters  of  the  Nashua  river;  southerly  by  the  Black- 
stone,  French  and  Quinebaug  rivers;    and  westerly 
and  southwesterly  by  the  Millers,  Ware,  Swift  and  Quaboag  rivers. 
These  trenches  are  the  pre-glacial  river  valleys,  and  are  partially 
filled  with  the  sands  and  gravels  of  the  Glacial  Period. 

This  plateau  is  an  ancient  peneplain ]  whose  general 
level  is  preserved  in  the  hills  of  1000-1100  feet 
elevation,  with  Wachusett,  Little  Wachusett,  Watatic, 
Mt.  Grace,  Asnebumskit  and  a  few  other  points,  constituting 
Monadnocks,  rising  above  the  general  level,  and  now  serving  as 
the  points  of  radiation  for  the  rivers  of  the  plateau,  just  as  they 
did  for  the  rivers  of  this  peneplain  when  it  was  at  base-level. 


1  Physical  Geography  of  Southern  New  England,  p.  276.— Wm.  M.  Davis. 


152  GEOLOGY   OF  WORCESTER. 

Worcester  lies  in  the  midst  of  this  plateau,  though 

Situation  of  ,      . 

Worcester        somewhat    near    the    eastern    border,    and    is    sunk 

in  the          about  500  feet  below  the  plateau  level.     Here  are 

found  most  of    the  rock  systems  which  form   this 

plateau,  and  under  such  conditions  as  to  be  specially  favorable 

for  study  and  the  determination  of  their  relations. 

Starting  at  Worcester,  as  a  centre,  we  find  first  and 

phyiiite         uppermost  the  Worcester  phyllite.1     This  rock  under- 

as  related  to      lieg  the  centrai  part  of  the  city,  and  may  be  traced 

the  plateau. 

southwesterly  and  northeasterly  across  the  state.  It 
presents  varying  stages  of  metamorphism  from  a  true  ar,gillite  to  a 
well  defined  mica  schist;  and  contains,  in  different  places,  min- 
erals resulting  from  metamorphism,  as  garnets,  chiastolite,  graphite, 
staurolite  and  anthracite.  The  laminae  of  the  phyllite  are  fre- 
quently highly  crumpled,  producing  folds  almost  infinitesimal  in 
size  and  infinite  in  number.  In  some  places  these  fine  folds  have 
been  so  compressed  and  flattened  that  they  constitute  the  beginning 
of  folia  of  a  new  structure  across  the  old.  In  one  place,  also,  this 
rock  exhibits  the  development  of  a  new  structure  by  the  rotation 
in  sections  of  the  laminae  between  fault  planes.2 

That  the  phyllite  is  really  uppermost  in  position  with  reference 
to  the  other  rocks  of  sedimentary  origin  found  in  Worcester  is 
shown  by  its  superposition  in  anticlines.  In  deciding  as  to  its 
position  in  the  geologic  series,  that  is,  its  age,  we  rely  on  the  spec- 
imens of  Lepidodendron  acuminatum  found  at  the  so-called  coal 
mine  here  in  Worcester;  and  we  assign  it  to  the  Carboniferous. 
It  probably  belongs  to  an  early  part  of  that  period. 

On  the  east,  this  phyllite  blends  into  a  rusty,  fibrolitic,  graphitic 
mica  schist  which  is  found  in  small,  detached  patches  within  the 
Bolton  gneiss  area.  These  are  probably  remnants,  indicating  the 
former  greater  extension  of  this  schist  formation. 

The  second  and  lower  formation  in  Worcester  is 
carboniferous     a   micaceous   quartzite 3  of   a   brownish   grey  color. 

quartzite  in 

the  plateau.       This  is  associated  with  the  phyllite  in  the  latter's 
extent  across  the   state,  and   occurs  in  bands  east 
and  west,  sometimes  also  in  the  midst,  of  the  phyllite. 


'Phyllite  is  used  as  defined  by  Merrill,  Smithsonian  Report,  U.  S.  National  Museum, 
1890,  p.  390.  Also  "  Rocks,  Rock- Weathering  and  Soils,"  p.  169.  This  phyllite  is  con- 
sidered in  detail  in  Chapter  I. 

*  Explained  on  page  7.     3  Considered  in  detail  in  Chapter  II. 


EDGE   OF  THE  PLATEAU  OF  CENTRAL  MASSACHUSETTS  FROM  THE 
SIDE  OF  COES  POND.     THE  PLATEAU  RISES  500  FEET  ABOVE 
THE  POND. 


EAST 


GEOLOGY   OF   WORCESTER.  153 

Because  of  the  close  relationship  which  these  two  formations 
bear  to  each  other,  the  micaceous  quartzite  is  also  assigned  to 
the  Carboniferous  period.  They  have  been  folded  together  on  a 
large  scale  in  the  Oakdale-Millstone  Hill  anticline,  the  phyllite 
above  and  the  quartzite  beneath;  and  they  have  been  crumpled 
together  on  a  small  scale  in  many  places,  as  may  be  seen  in  the 
rocks  of  the  deep  cuts  of  the  Boston  and  Albany  and  Boston  and 
Maine  railroads  here  in  Worcester.  These  two  formations  appear, 
as  far  as  can  be  seen  in  the  midst  of  such  severe  folding,  to  be 
conformable. 

Like  the  phyllite,  the  quartzite  also  shows  the  development  of 
a  new  or  secondary  structure  across  the  old  or  original  structure; 
and  in  the  case  of  the  rock  of  Wigwam  Hill  the  new  structure  was 
formed  by  the  compression  of  small  folds  in  the  micaceous  part 
of  the  quartzite,  so  that  these  compressed  folds  have  become  the 
folia  of  the  schist  as  it  is  at  the  present  time.  This  quartzite  is 
in  a  few  places  conglomeratic,  and  the  pebbles  show  deformation 
by  pressure,  being  more  or  less  flattened  in  the  plane  of  the 
laminae. 

Penetrating  the  Carboniferous  phyllite  and  quartz- 
16  ite  are  granite  bosses,  of  which  Millstone  Hill  *  is  typi- 
cal. These  granites  are  later  than  the  Carboniferous. 
That  of  Millstone  Hill  contains  inclusions  of  both  the  phyllite  and 
quartzite.  The  structure  of  the  adjoining  Carboniferous  quartzite 
wraps  around  this  granite,  sometimes  with  the  original  bedding 
and  sometimes  across  it.  The  rocks  in  the  immediate  vicinity  of 
the  granite  have  been  greatly  shattered,  and  now  frequently  appear 
as  breccias,  or  made  up  of  small  angular  blocks  cemented  together 
by  fine  quartz  veins.  Even  the  phyllite  included  in  the  granite 
is  brecciated,  indicating  that  the  pressure  was  not  excessive  when 
this  was  included  in  the  molten  granite,  hence  that  this  granite 
did  not  solidify  at  a  very  great  depth  beneath  the  surface  of  the 
earth. 

This  granite  of  Millstone  Hill  contains  orthoclase,  generally 
white,  with  a  triclinic  feldspar,  also  white  in  color;  quartz,  gen- 
erally smoky,  but  sometimes  blue  or  amethystine;  and  a  small 
amount  of  biotite.  The  quartz  is  distributed  in  somewhat  coarse, 
granular  particles  outside  of  the  feldspar,  also  in  rounded  grains, 
and  rarely  in  small  bi-pyramidal  crystals,  included  in  the 

1  Considered  in  detail  in  Chapter  III. 


154  GEOLOGY   OF    WORCESTER. 

feldspar.  Fluorite  also  occurs  in  small  particles  as  an  original 
mineral  in  this  granite.  Beryl  occurs  in  approximately  spherical 
segregated  masses,  a  foot  or  so  in  diameter,  irregularly  distri- 
buted through  the  mass  of  the  granite.  Other  segregated  masses 
also  occur  containing  beryl,  garnets,  fluorite,  molybdenite,  sphal- 
erite, pyrite  and  a  light  colored  mica. 

This  granite  boss  is  cut  in  various  directions  by  aplite  dikes 
varying  in  width  up  to  twenty  feet.  A  part  of  this  aplite  has 
been  brecciated.  The  upcoming  of  this  granite  was  probably  as- 
sociated with  the  folding  of  the  phyllite  and  quartzite,  as  the  boss 
is  situated  at  the  southern  terminus  of  the  Oakdale-Millstone  Hill 
anticline.  There  are  other  granite  areas  distributed  along  this 
anticline. 

The  Carboniferous  quartzite,  traced  to  the  east, 
in'the'pfateau  blends  into  a  coarser  grained,  more  highly  meta- 
morphosed quartzose  mica  schist,  frequently  con- 
taining alternating  hornblendic  bands.  Between  the  laminae  of 
this  schist  there  has  been  forced,  by  parallel  injection,  much  coarse 
granite.  This  schist  and  granite  afford  many  minerals,  both  origi- 
nal and  secondary,  as  enumerated  in  the  study  of  the  quarry  near 
Quinsigamond.  The  alternation  of  schist  and  granite  is  very 
gneissoid  in  appearance,  and  often  presents  a  close  resemblance 
to  a  metamorphic  conglomerate.  From  this  appearance,  and  from 
the  fact  that  it  extends  through  the  town  of  Bolton,  and  there 
contains  the  well  known  limestone  mineral  locality,  this  phase 
of  the  Carboniferous  quartzite  is  called  the  Bolton  gneiss.1 

It  is  upon  this  Bolton  gneiss  that  are  spread  the  patches  of 
the  rusty,  fibrolitic,  graphitic  mica  schist  phase  of  the  Carbonifer- 
ous phyllite  already  referred  to  as  indicating  the  former  greater 
extension  of  that  formation. 

Within  this  Bolton  gneiss  are  also  small  areas  of  crystalline 
limestone  in  the  towns  of  Boxboro,  Bolton,  Northboro,  Millbury 
and  Webster.  These  crystalline  limestones  abound  in  minerals 
formed  during  the  metamorphism,  and  the  Bolton  and  Boxboro 
limestones  have  long  been  noted  for  the  scapolite  and  other  min- 
erals they  afford. 

Diorite  in  Dikes  of  diorite  2  are  found  in  the  Bolton  gneiss  in 

Boiton  gneiss      the  eastern  part  of  Worcester  and  in  Shrewsbury. 

This  diorite,  or  these  diorites  are  of  interest  as  repre- 

»  Considered  in  detail  in  Chapters  IV.  and  VI.    *  Ibid.,  V.  and  VI. 


WlLBRAHAM     MOUNTAINS,    FROM    LEVEL    FLOOR    OF    THE    CONNECTICUT 
VALLEY. 


GEOLOGY    OF   WORCESTER.  155 

senting  the  basic  eruptives  in  this  study,  and  also  on  account  of 
the  minerals  they  afford,  and  the  mineral  transformations  they 
present. 

The  Bolton  gneiss  may  be  traced  northeasterly, 
BoMxmgMta.  easterly  and  southeasterly  from  Worcester  and  is 
found  underlying  Oxford,  Millbury,  Graf  ton,  Shrews- 
bury, Northboro,  Berlin,  Bolton,  Boxboro  and  other  towns  to 
the  northeast.  It  is  seen  from  this  that  the  Bolton  gneiss 
makes  up  a  large  portion  of  the  eastern  part  of  the  plateau  of 
Central  Massachusetts. 

Best  direction         If,  in  tracing  the  Bolton  gneiss,  we  go  east  from 

in  which  to       Worcester  on  the  Boston  and  Albany  railroad,  be- 

from  tween  North  Grafton  and  Westboro,  we  pass  from 

Worcester.  fae  gneiss  on  to  a  peculiar  amphibolite.  This  latter 
rock  belongs  with  the  rocks  of  eastern  Massachusetts  rather  than 
with  those  of  the  plateau,  and  does  not  help  in  our  study,  but 
rather  confuses  it,  because  this  rock  separates  the  Bolton  gneiss 
along  this  line  from  the  rocks  of  the  eastern  border  of  the  plateau. 
It  is  well  for  us,  therefore,  in  tracing  the  Bolton  gneiss,  to  follow 
a  southeasterly  direction  through  the  town  of  Millbury  into  the 
town  of  Sutton,  and  thus  avoid  the  peculiar  amphibolite,  which 
does  not  reach  southwesterly  so  far  into  the  plateau  region. 

In  this  direction  we  find  that  the  Bolton  gneiss  extends  from 
Worcester  through  Millbury  and  the  northwestern  part  of  Sutton; 
then  as  we  go  still  farther  to  the  southwest,  in  the  vicinity  of  West 
Sutton,  we  become  aware  that  the  rock  beneath  is  quite  different. 
This  change  from  one  formation  to  another  may  be  nicely  seen 
about  three  fourths  of  a  mile  southwest  of  West  Sutton. 

Westboro  This  more  ancient  formation  consists,  generally,  of 

^n'the*  a  lignt  colored,  nearly  white,  finely  grained,  sugary 
plateau.  quartzite  which  is,  at  times,  actinolitic.  It  con- 
stitutes a  co  nparatively  narrow  band  extending  through  the  towns 
of  Webster,  Oxford,  Sutton,  Grafton  and  Westboro;  and  is  called 
the  Westboro  quartzite.1  As  it  dips  beneath  the  Bolton  gneiss 
on  the  western  side  of  a  large  anticline,  the  quartzite  is  con- 
sidered older  than  the  gneiss.  In  appearance  this  quartzite  reminds 
one  of  the  Cambrian  quartzite  of  Western  Massachusetts,  though 
there  is  nothing  in  this  eastern  rock  to  definitely  fix  its  geologi- 
cal age.  

1  Not  before  considered  because  not  in  Worcester. 


156  GEOLOGY   OF   WORCESTER. 

Anticline  The    anticline,    on    whose    western    flank    is    this 

making  the       quartzite,   is  a  broad    one  extending  from    Rhode 

southeastern  i        i      •  -»«•  i  i         i 

border  of  Island  into  Massachusetts  through  the  towns  of 
the  plateau.  Douglas  and  Uxbridge,  Sutton  and  Northbridge, 
Grafton  and  Upton,  Westboro  and  Southboro,  to  the  road  be- 
tween Wessonville  and  Fayville  in  the  last  two  towns,  where  it 
suddenly  ends  at  a  fault.  This  broad  anticline  pitches  north- 
easterly. The  rock  of  this  anticline  has  been  called  the  North- 
bridge  gneiss.1 

Northbridge          ^his  Northbridge  gneiss  is  of  a  light  grey  color,  tinted 
gneiss  in  the      flesh  red  by  the  feldspar.  It  is  also  of  a  medium 

plateau.         coarse,  granular  texture,  and  of  foliated  structure. 

The  feldspar  and  quartz  in  this  gneiss  are  granulated.  The 
indistinct  foliation  is  apparently  but  the  crushing  and  flattening 
out  by  pressure  of  quartz  and  feldspar  particles  into  a  plane  at 
right  angles  to  the  direction  of  the  crushing  force.  The  biotite 
occurs  in  very  thin,  indistinct,  bladed  units,  a  half  inch  to  an 
inch  long  and  an  eighth  to  a  quarter  of  an  inch  wide;  and  each 
unit  is  made  up  of  many  little,  black  scales  lying  parallel  to  the 
foliation. 

This  Northbridge  gneiss  looks  as  if  it  was  once  porphyritic  and 
massive,  and  had  become  granular  and  foliated  through  crushing. 
In  some  places  we  may  find  the  uncrushed  centres  of  the 
feldspar  individuals,  which  are  probably  the  remnants  of  the 
porphyritic  crystals.  Magnetite  is  always  found  in  this  rock, 
generally  in  fine  irregular  grains,  sometimes  as  octahedra  of 
considerable  size. 

This  Northbridge  gneiss,  making  up  the  anticline  already  de- 
scribed, and  dipping  easterly  and  westerly  under  the  adjacent 
formations,  is  considered  the  most  ancient  of  the  rocks  of  the 
plateau  of  Central  Massachusetts.  This  rock  makes  up  the  south- 
eastern border  of  this  plateau  in  the  towns  of  Douglas,  Uxbridge, 
Northbridge,  Upton  and  Grafton. 

We  have  now  briefly  considered  the  rocks  from 
Worcester  to  the  eastern  and  southeastern  border 


from  of  the  plateau   of  Central   Massachusetts,   bringing 

out  the  relation  of  each  to  the  succeeding  one,  as 

far  as  we  can;  let  us  next  start  from  Worcester,  and,  in  like  man- 

ner, trace  the  rocks  westerly.     Passing  over  the  phyllite,  which 

1  Not  before  considered  because  not  in  Worcester. 


WORCESTER  AS  SEEK,  LOOKING  EAST,  FROM  THE  EDGE  OF  THE  PLATEAU 

OF  CENTRAL  MASSACHUSETTS.    THE  DOME-SHAPED  HILLS  ARE 

DRUMLINS. 


GEOLOGY   OF   WORCESTER.  157 

is  found  underlying  the  central  part  of  Worcester,  and  which  has 
already  been  considered,  we  find  in  the  western  part  of  the  city 
the  Carboniferous  micaceous  quartzite,  identical  with  that  found 
east,  and  in  the  midst,  of  the  Carboniferous  phyllite.  This 
arrangement  of  these  two  formations  is  due  to  folding  and  sub- 
sequent erosion,  by  which  the  lower  one  is  revealed  alongside 
of  the  upper. 

Paxton  OQ  f°ll°wmg  this   micaceous  quartzite  in  the  ex- 

schist  in         treme  western  part  of  Worcester  the  rock  becomes 

the  plateau.  coarser  jn  grain  and  abounds  in  parallelly  in- 
jected granite,  giving  an  appearance  that  frequently  simulates  a 
metamorphic  conglomerate.  This  western  and  more  highly  meta- 
morphosed phase  of  the  Carboniferous  quartzite  has  been  called 
the  Paxton  schist,1  from  its  occurrence  in  that  town,  just  as  the 
similar  gneissoid  extension  of  the  same  quartzite  on  the  east  is 
called  the  Bolton  gneiss.  The  Paxton  schist  is  less  gneissoid 
than  the  Bolton  gneiss  because  of  a  smaller  proportion  of  injected 
granite. 
Brimfieid  Lying  above  the  Paxton  schist,  as  is  shown  by 

schist  in  the  its  upper  position  in  an  anticline  in  the  northwestern 
part  of  Worcester,  is  a  rusty,  graphitic,  fibrolitic 
mica  schist3  identical  with  the  rusty,  graphitic,  fibrolitic  mica 
schist  found  in  patches  within  the  area  of  the  Bolton  gneiss.  This 
rusty  schist  has  been  crumpled  into  almost  innumerable  folds, 
both  large  and  small,  and  contains  much  injected  granite.  Lying 
as  it  does  above  the  Paxton  schist,  which  is  a  more  highly  meta- 
morphosed phase  of  the  Carboniferous  quartzite,  this  rusty  schist 
bears  the  same  relation  to  the  Paxton  schist  that  the  Worcester 
phyllite  bears  to  the  Carboniferous  quartzite;  also  on  the  east 
side  of  Worcester  there  is,  in  a  small  area,  a  transition  from  the 
phyllite  into  a  like  rusty,  graphitic,  fibrolitic  schist;  this  rusty 
schist  in  the  western  part  of  Worcester,  called  the  Brimfieid  schist, 
is,  then,  a  more  highly  metamorphosed,  or  more  coarsely  crys- 
tallized, phase  of  the  Worcester  phyllite,  and  belongs  to  the  Car- 
boniferous period.  This  Brimfieid  schist,  in  addition  to  making 
up  large  areas,  also  occurs  in  limited  patches  within  broad  areas 
of  the  Paxton  schist.  These  patches  probably  indicate  the  for- 
mer extension  of  this  upper  Brimfieid  schist,  and  are  remnants 
that  have  escaped  in  the  profound  erosion  by  which  the  rock  sur- 

i  Considered  m  detail  in  Chapter  VII.    »  Ibirl. 


158  GEOLOGY   OF   WORCESTER. 

face  of  this  plateau  has  been  formed.  These  two  rocks,  the  Pax- 
ton  and  Brimfield  schists,  together  with  their  included  eruptives, 
make  up  the  plateau  of  Central  Massachusetts  westerly  from 
Worcester  to  the  boundary  as  already  defined. 

But  these  Carboniferous  rocks  have  even  a  greater 
u^carbon*  extension  to  the  south  and  north,  and  may  be  traced 
iferous  rocks,  far  into  Connecticut  on  the  one  hand,  and  beyond 

the  Massachusetts  boundary  into  New  Hampshire 
on  the  other.  So  far-reaching  is  the  study  on  which  we  enter 
when  we  begin  that  of  the  rocks  of  Worcester.  In  the  parlance 
of  war,  Worcester  occupies  the  strategic  point. 
Time  of  forma-  While,  from  what  has  been  said,  it  may  be  seen 
tionofthe  that. the  plateau  of  Central  Massachusetts  is  made 

up  of  Carboniferous  rocks,  the  formation  of  this 
plateau,  or  of  the  peneplain  which  is  the  plateau  surface,  was 
much  later,  since  the  Juratrias  rocks  of  the  Connecticut  valley 
were  in  their  present  position  before  this  peneplain  was  formed 
by  the  rivers  cutting  this  region  down  to  base  level,  and  the  sub- 
sequent elevation  of  the  land  to  the  present  altitude  of  the  plateau- 
Being  later  than  the  Juratrias  the  formation  of  the  peneplain 
has  been  assigned  to  the  Cretaceous, l  and  the  subsequent  eleva- 
tion, during  which  the  rivers  of  the  plateau  cut  the  trenches  which 
they  now  follow  in  this  plateau,  to  the  Tertiary. 

But  reaching  farther  back  in  geologic  time  than 
Juratrias  land  ^he  plateau  or  the  peneplain  constituting  the  plateau 
plateau  region,  surface,  are  the  Monadnocks,  Asnebumskit,  Wachu- 

sett,  Watatic,  Mt.  Grace,  and  others,  rising  in  each 
case  several  hundred  feet  above  the  plateau  level.  These  tell  of 
another  land  surface,  a  thousand  feet  or  so  above  the  present 
plateau,  very  likely  another  peneplain  and  an  earlier  plateau. 
As  the  rocks  in  these  elevations  are  the  Carboniferous  schists,  or 
their  included  eruptives  already  spoken  of,  this  more  ancient  land 
surface  must  have  been  made  out  of  Carboniferous  rocks,  and 
hence  must  have  been  later  than  the  Carboniferous  period,  and . 
earlier  than  the  Cretaceous.  It  may  then  be  assigned  to  the 
Juratrias.  The  meaning  of  all  this  is  that  the  Carboniferous  rocks 
of  the  present  time  are  but  a  part,  perhaps  a  small  part,  of  the 
original  Carboniferous  rocks;  that  these  rocks  extended  to  a  con- 


i  Win.  M.  Davis,  18th  Annual  Kept.  U.  S.  Geol.  Survey,  Part  II.,  p.  14. 


MOUNT  WACHUSETT,  RISING  NEARLY  1000  FEET  ABOVE  THE  SURROUNDING 
PLATEAU. 


GEOLOGY   OF   WORCESTER.  159 

siderable  height  above  the  tops  of  the  Monadnocks,  and  that 
during  the  Juratrias  period  these  rocks,  including  the  eruptives, 
were  profoundly  eroded  by  the  rivers,  furnishing  materials, — sands, 
clays,  and  pebbles, — which  were  deposited  during  that  same 
period  in  the  Connecticut  valley  region,  which  was  then  an  arm 
of  the  sea,  and  in  other  regions  whither  the  rivers  carried  them. 
Such,  in  brief,  is  the  study  of  the  rocks  of  the  plateau  of  Cen- 
tral Massachusetts,  sunk  in  which  is  the  broad  Tertiary  valley 
in  which  Worcester  is  situated. 


Actinolite,  100,  103,  108. 
Adams  square,  Phyllite  at,  4. 
Agassiz,  Louis,  18,  29. 
Age  of  Bolton  gneiss,  128. 

Brimfield  schist,  148. 

Granite,  60. 

Micaceous  quartzite,  127. 

Paxton  schist,  139. 

Phyllite,  29, 

Quartzite,  50. 

Agents  of  metamorphism,  20. 
Algonkian,  28. 
Allanite,  94. 
Amphibolite,  155. 
Andalusite  in  mica  schist,  122. 
Andalusite  phyllite,  6,  27. 
Andrews,  Calvin  H.,  18. 
Anhedra  of  quartz,  54. 
Ankerite,  24,  72. 
Anticlinal  fold,  146. 
Anticline,  43,  44,  57,  124,  126,  127, 

147,  153,  156. 
Apatite,  118,  138. 
Aplite,  9,  62,  63,  64,  65,  66,  67,  78, 

117,  154. 
Archean,  28. 

Asnebumskit,  143,  151,  158. 
Assabet  river,  151. 
Auburn,  4,  25,  26,  27,  136. 
Azurite,  100. 

Ballard  field,  116-129. 

Ballard's  quarry,  80. 

Banded  granite',  83. 

Banding  in  rocks,  38,  83,  90,  132. 

Basic  eruptive,  120. 

Basic  rock,  114. 

Batholite,  92,  140. 

Beds,   Determination  of  upper  and 

lower,  124,  126. 

Bell  pond,  Aplite  on  shore  of,  64. 
Berlin,  155. 
Beryl,  67,  68,  154. 
Biotite  gneiss,  82,  84. 
Biotite  in  granite,  54. 
Biotite  schist,  82,  84,  86. 
Blackstone  river  valley,  151. 
Bloomingdale,  48. 
Blue  quartz,  53. 
12 


Bolton,  104,  154,  155. 
Bolton  gneiss,  79. 

Conformable  with  mica  schist,  128. 

Carboniferous,  129. 

Description  of,  80. 

Dip  of,  79,  80. 

Extent  of,  155. 

in  plateau  of  Central  Mass.,  154. 

Relation     of,     to     Carboniferous 
quartzite,  116,  129. 

Relation  of,  to  Carboniferous  rocks, 
122. 

Relation  of,  to  mica  schist,  128. 

Relation  of,  to  Paxton  schist,  139. 

Strike  of,  79,  80. 
Boxboro,  104,  154,  155. 
Boylston,  5. 

Breccia,  16,  40,  41,  61,  66. 
Brecciation    of    rocks    in    Millstone 

Hill  area,  153. 
Brimfield  schist,  141,  157. 

Age  of,  148,  157. 

Anticlines  in,  147. 

at  the  Cascade,  142. 

Boundary  of,  142. 

described,  141. 

Dip  of,  145. 

Extension  of,  142. 

Faults  in,  148. 

Fibrolite  in,  142,  145. 

Folding  of,  157. 

Former  extent  of,  157. 

Garnets  in,  142. 

Graphite  in,  142,  145. 

near  end    of    North    Bend  street, 
145. 

Relation  of,  to  Paxton  schist,  143, 
146. 

Relative  position  of,  157. 

Remnants  of,  144. 

Scattered  areas  of,  149. 

Strike  of,  145. 

Thickness  of,  147. 

Tourmaline  in,  142. 
Burncoat  street,  36. 

Calcite,  24,  37,  72,  96. 
Cambrian,  28. 
i  Carbonate  of  iron  and  calcium,  24. 


162 


INDEX. 


Carboniferous,  29,  51,  127,  137,  139, 

149,  157. 
Carboniferous   rocks,    Extension    of, 

158. 

Former  thickness  of,  158. 
Cascade,  136,  142,  143,  144. 
Cavities  in  aplite  and   quartzite,  37, 

66. 
Central   Massachusetts,    Plateau    of, 

151,  158. 
Rocks  of,  149. 
Chabazite,  97. 
Chadwick  street,  31,  33. 
Chalcopyrite,  108. 

Chandler  street,   Quartzite  in,    137. 
Charlton  station,  Ledges  near,  140. 
Chiastolite,  22,  28. 
Chlorite,  83. 

Chlorite  biotite  schist,  89. 
City  Hall,  40. 
Cleavage,  Slatv,  8,  26. 
Coal,  17. 

Coal  mine,  14,  16,  18,  47,  56. 
Compass,  Variation  of,  5. 
Complex  compounds,  71. 
Compression,  90. 
Concentration  of  minerals,  73. 
Conglomerate,  82,  88,  89,  157. 
Connecticut,   158. 
Connecticut  valley,  151,  159. 
Contact  of  fusion,"  59,  64. 
Copper  pyrites,  100,  108. 
Court  Hill,  Phyllite  in,  3. 
Cretaceous,  29,  158. 
Crumpling  of  quartzite,  24,  45. 
Crushed  border  of  feldspar,  81. 
Crushed  feldspar,  84. 
Crushed  granite,  35,  135. 
Crushing,  62,  76,  108. 
Crystalline  aggregates,  72. 
Crystallization,  19,  71,  85,  87. 

Deep  Cut,  B.  *  A.  R.  R.,  4,  5,  8,  127. 
Devonian,  29. 

Dike,  56,  64,  105,  112,  114. 
Diorite,   105-115,  118-120,  154. 
Dip,  5,  23,  31,  35,  37,  38.  39,  40,  42, 

43,  45,  79,  80,  130,  134,  137,  138. 

141,  145. 
Disintegration    produced    by    fluor 

spar  and  iron  pyrites,  77 
Dodge  Park,  34,  50.' 
Dolomite,  110,  111. 
Douglas,  156. 
Drumlin,  1. 
Dudley,  4,  136. 

Eakins,  L.  G.,  18. 

Earth's  surface,  Lowering  of,  115. 


East  Central  street,  52. 

East  Kendall  street,  Rocks  in,  39, 

56,  57,  60. 

East  Worcester,  39,  40. 
Elements,  Union  of,  70. 
Elm  street,  Phyllite  in,  4. 
Eocene,  29. 
Epidote,  99. 
Erosion,  Granite  evidence  of,  60. 

revealing  granite,  78. 
Extension  of  quartzite,  34. 

Fault,  23,  33,  47,  147,  148,  156. 

Faulting,  45,  152. 

Fayville,  156. 

Feldspar,  36,  53,  63,  81,  84,  103,  108, 

114. 

Fetid  aplite,  67. 
Fibrolite,    101,    105,    109,    122,    123 

142,  145. 
Fissures,  75,  76. 

Fluor  spar,  55,  73,  69,  77,  154. 
Fold,  39,  40,  44,  48,  124,  127. 
Folded  quartz  veins,  23. 

quartzite,  123. 
Folding,  23,  24,  45,  46,  84,  90,  91, 

92,  93,  124,  152. 
Foliated  granite,  135. 
Foliation,  76,  106,  156. 
Fossils,  18. 

Fourth  outcrop  of  diorite,  109. 
Fowler  street,  Granite  in,  135. 
French  river,  151. 
Frost,  Action  of,  75,  131. 

Garnet,  22,  27,  68,  82,  94,  103,  IIS, 
120,  121,  122,  123,  132,  133,  142, 
154. 
Garnetiferous  gneiss,  96. 

mica  schist,  95. 
Geological  ages,  28. 
Geological  map,  4. 
George  Hill,  28. 
Gibbs  street,  116,  118. 
Glacial  marks  or  striae,  12,  41,  118, 

134. 

Glacial  period,  78,  134. 
Gneiss,  Biotite,  82,  84. 

Foliated,  79. 

Garnetiferous,  96. 

Hornblende,  82. 

Northbridge,  156. 
Gneissoid  granite,  81,  82,  84,  86. 
Grafton,  155,  156. 
Granite,  Action  of  frost  on,  75. 

Age  of,  60. 

Ankerite  in,  72. 

Apatite  in,  138. 

Aplite  dikes  in,  117. 


INDEX. 


163 


Granite,  Banding  in,  83,  85. 

Beryl  in,  67,  (58. 

Brownish  grey,  85,  87,  93. 

changed  by  phyllite,  59,  74. 

Coarse,  light  grey,  93. 

Cracks  in,  62. 

Crushed,  35,  117,  135. 

Crushing  of,  76. 

Dark  grey,  73. 

Depth  at  which  it  solidified,  61, 
62. 

Dikes  of,  112. 

Disintegration  of,  by  combined  ac- 
tion of  fluor  spar  and  iron  py- 
rites, 77. 

Effect  of,  on  neighboring  rocks,  56. 

Effect  of,  on  thickness  of  forma- 
tion, 92. 

Effect  of  quartzite  on,  60. 

Essential  minerals  of,  55. 

Evidence  of  erosion,  60. 

Extent  downward  of,  7S. 

Fissures  in,  75. 

Fluor  spar  in,  69. 

Foliated,   117,  135. 

Foliation  of,  76. 

Garnetiferous,  68,  121,  133. 

Gneissoid,  80,  81,  82,  84,  86. 

Injected,  89. 

in  micaceous  quartzite,  123. 

in  Paxton  schist,   136. 

impregnating  phyllite,  74. 

in  Providence  street,  116. 

in  schist,  88,  93. 

in  Tatnuck  Hill,  135. 

Iron  pyrites  in,  69. 

Jointing  of,  75. 

Light  grey,  83. 

Mineral  locality  in,  6S. 

Minerals  in,  of' Millstone  Hill,  153. 

Molybdenite  in,  68. 

of  Millstone  Hill,  52. 

Parallelly  injected,  154. 

Phyllite  inclusion  in,  74. 

porphyry,  13. 

Proof  that,  is  an  eruptive,  55. 

Quartz  crystals  in,  72. 

Quartzite  inclusion  in,  59,  117. 

Relation  of,  to  aplite,  64. 

Relation  of,  to  quartzite,  39. 

Rounding  of  corners  of,  77. 

Segregation  area  in,  69. 

Sheared,  117. 

Sheets  in,  75. 

Solidified  in  the  earth,  60. 

Sphalerite  in,  68. 

Summary  of  facts  relating  to,  78. 

Three-mica,  135. 

Tourmaline,  130,  132,  142,  147. 


Granite,  Weathering  of,  76. 
Granulation  of  feldspar,  108,  114. 
Graphite,  6,  8,  16,  17,  22,  61,  95,  102, 

122,  127,  142,  145,  150. 
Graphitic  acid,  16. 
Gravels,  Thickness  of,  1. 
Green  Farm,  38,  52. 
Grove  street,  Quartzite  in,  31. 
Gummite,  94. 

Heat  in  metamorphism,  20. 
Hey  wood  Farm,  Diorite  on,  120. 
Highland  street,  Quartzite  beneath, 

31. 

Hitchcock,  Dr.,  16. 
Holden  Reservoir,  27. 
Hornblende,  11,  81,  82,  94,  95,  105, 

106,  107,  111,  118,  132,  139. 
Hornblende  schist,  87,  105,  106,  107, 

113,  114,  132. 
Hunt  street,  Quartzite  at  corner  of, 

40. 

Inclusion,  58,  59,  65,  84,  117. 

Iron,  70. 

Iron  carbonate,  72. 

Iron  pyrites,  7,  15,  34,  63,  69,  73,  77, 

100,  108,  119,  122,  134,  154. 
Iron  rust,  75,  77. 

Jointing  of  granite,  75. 
Joints,  52,  114,  131. 
Juratrias,  29,  158,  159. 
Juratrias  plateau,  158. 

King,  Homer,  141. 

Lake  View,  27. 

Laminae,  5,  7,  38. 

Lamination  of  Paxton  schist,  131. 

Lancaster,  5,  28. 

Layers,  Original,  of  phyllite,  22. 

Ledges.  1,  3. 

Leominster,  151. 

Lepidodendron  acuminatum,   152. 

Liberty  Farm,  143. 

Limestone,  101,  102,  103,  104,  154. 

Lincoln  street,  4,  37. 

Little  Wachusett,  151. 

Lunenberg,  151. 

Lyell,  Sir  Charles,  29. 

Maclnnes,  John  C.,  27. 
Magma,  55,  69,  70,  71,  78. 
Magnetic  pyrites,   98,    119. 
Magnetite,  103,  109,   110,  122,   156. 
Malachite,  100. 
Manganese  carbonate,  72. 
oxide,  77. 


164 


INDEX. 


Metamorphic  conglomerate,  82. 
Metamorphism,  19,  20,  21,  33,  46. 
Mica,  21,  28,  36,  76,  84. 
Mica  schist,  3,  26,  88. 

above  the  quartzite,  126. 

Andalusite  in.  122. 

Carboniferous,  127. 

Conformable   with   Bolton   gneiss, 
128. 

Fibrolitic,  100,  105,  109,  122,  123. 

Garnets  in,  122,  123. 

Graphite  in,  95. 

Hornblende  in,  81. 

in  Ballard  field,  122,  123. 

Origin  of,  87. 
Mica  schist,  Phyllite  in,  128. 

Quartzose,   130. 

Relation  of,  to  Bolton  gneiss,  128. 

Relation  of,  to  quartzite,  123,  124, 
126. 

Rusty,  107,  112,  113,  114,  122,  141, 
149.' 

Rusty,    graphitic,    fibrolitic,    152, 

154,  157. 

Micaceous  quartzite,  8,  12,  157 
Mill  street,  Ledge  in,  138. 
Millbury,  154,  155. 

Bolton  gneiss  in,  104. 

Limestone  in,  101,  102. 

Vermiculite  in,  99. 
Millers  river,  151. 
Millstone  Hill,  38,  39,  41,  42,  44,  52, 

57,  78,  92,  153. 

Millstone  Hill— Oakdale  anticline,  44. 
Mineral  locality,  68,  93. 
Minerals,  73,  96. 
Moen,  Mr.,  Estate  of,  113. 
Moisture  in  metamorphism,  21. 
Molybdenite,  68,  154. 
Monadnocks,  151,  158. 
Monoclinic  feldspar,  53. 
Mt.  Grace,  151,  158. 
Muscovite,  54. 

Nashua  river,  43,  151. 

Neocene,  29. 

New  Hampshire,  158. 

Newton  Hill,  31. 

Nickel  in  pyrrhotite,  98. 

Normal  School  Hill,  39,  40,  52,  62, 

76. 

North  Bend  street,  145. 
Northboro,   104,   151,   154,   155. 
Northbridge,  156. 
Northbridge  gneiss,  156. 
North  Park,  35,  36,  50. 
Nye,  Mr.,  27. 

Oak  Hill,  4. 


Oakdale,  49. 

Oakdale  anticline,  43,  57,  153. 

Odor  in  aplite,  67. 

Olivine,  110,  111. 

Original  minerals,  96. 

Original  structure,  47. 

Ottrelite,  40,  41. 

Oxford,  4,  136,  155. 

Pakachoag  Hill,  4. 
Paxton  schist,   132,  157. 

Banding  in,  132. 

described,  131. 

Dip  of,  130,  134,  138,  141,  145. 

Extension  of,  134,  136. 

Fault  in,  147,  148. 

Garnets  in,  132. 

Granite  in,  136,  139. 

highly    crystallized    Carboniferous 
quartzite,  139. 

in   Central   Massachusetts,    149. 

Iron  pyrites  in,  134. 

Joints  in,  131. 

Lamination  of,  131. 

made  from,  141. 

near  North  Bend  street,  145. 

No  limestone  in,  140. 

Pseudo-conglomerate,  140. 

quartose  mica  schist,  130. 

Relation  of,  to  Bolton  gneiss,  139, 
140. 

Relation   of,   to   Brimfield   schist, 
143,  144,  146. 

Relation     of,     to     Carboniferous 
quartzite,  137,  139. 

Splitting  of,  131. 

Stilbite  in,  134. 

Strike  of,  130,  134,  138,  141,  145. 

Tourmaline  granite  in,  132. 
Pebble-like  inclusions,  84. 
Pelham  Hills,  151. 
Peneplain,  151. 
Pennsylvania,  29. 
Phyllite,  3-29,  152. 

Age  of,  28,   152. 

at  the  Summit,  23. 

breccia,  61. 

containing  andalusite,  27. 

defined,  3. 

described,  6. 

Dip  of,  35,  43. 

Disturbance  of,  78. 

Effect  of,  on  granite,  59,  74. 

Extent  of,  4,  152. 

Finely  ribbed,  2o. 

Folding  of,  23. 

Former  state  of,  19. 

Graphitic,  128. 

impregnated  with  granite,  74. 


INDEX. 


165 


Phyllite  in  mica  schist,  128. 

Inclusions  of,  58,  61,  65,  74. 

Metamorphic  phases  of,  152. 

Metamorphism  of,  19 

Minerals  in,  153. 

New  structure  in,  7,  26. 

Original  layers  of,  22. 

Position  of,   152. 

Quartzite  folded  into,  127. 

Relation  of,  to  quart  Kite,  48. 

Remnants  of,  44. 

removed  by  erosion,  57. 

Secondary  structure  in,  7,  26,  152. 

Strength  of,  61. 

Strike  of,  15,  35,  43. 

Thickness  of,  25,  49. 

Weathering  of,  10. 
Pinching  out  of  quartzite  bands,  45. 
Pitch  of  anticline,  127. 
Plagioclase,  108. 
Plateau,  Ancient,  158. 
Plateau    of    Central    Massachusetts, 

134,  149,   151,   158. 
Plantation  street,  4,  12,  41,  48. 
Plastic  state  of  rocks,  46. 
Pleasant  street,  4,  30. 
Pleistocene,  29. 
Pom  fret,  4. 
Porphyry,  Granite,  13. 
Prehnite,  97. 

Pressure,  23,  20,  60,  71,  76. 
Prochlorite,   17,  99. 
I'rovidence  street.  4.  116. 
Pseudo-conglomerate,   88,   89,    140. 
Putnam,  Gen.  Israel,  5. 
Pyrites,  Magnetic,  98. 
Pyroxene,  103. 
Pyrrhotite,  98,   108,   119,   122. 

Quaboag  river,  151. 

Quartz,  13,  53,  54,  72,  73,  81,  96,  153. 

Quartz  veins,  6,  14,  23,  34,  36.  37 

42,  58,  65,  83,  98,  100. 
Quartzite  in  aplite,  66. 
Quartzite,  Micaceous,  8,  12,  152,  157. 

Age  of,  50. 

Anticline  in,  43,  126,  127. 

at  the  Summit,  24. 

Breaking  of,  55. 

Calcite  in,  37. 

Cavities  in,  37. 

Condition  of,  during  folding,   46. 

Conglomeratic,  153. 

Description  of,  30,  31. 

Dip  of,  13,  36,  37,  39,  42,  43,  45. 

Disturbance  of,  78. 

Extent  of,  31,  34,  48,  152. 

Fault  in,  33. 

Folded,  39,  123. 


Quartzite,  Folding  of,  45. 

Granite  in,   123. 

in  Ballard  field,  122. 

in  Dodge  Park,  34. 

in  East  Kendall  street,  39. 

in  Hunt  street,  40. 

in  Lincoln  street,  37. 

in  North  Park,  36. 

Inclusions  of,  59,  117. 

Interfolding  of,  153. 

Lamination  of,  32. 

Metamorphism  of,  33. 

near  Coal  Mine,  14,  41. 

near  B.  &  M.  R.  R.,  42. 

Origin  of,  32. 

Original  state  of,  46. 

Pliable,  45. 

Position  of,  48. 

Quartz  veins  in,  42. 

Relation  of,  to  Bolton  gneiss,  129. 

Relation  of,  to  granite,  39,  56. 

Relation  of,  to  mica  schist,   123, 
124,  126. 

Relation  of,  to  phvllite,  49,  127, 
153. 

Secondary  structure  in,  153. 

Strike  of,'  13,  36,  37,  39,  42,  43,  45. 

Structure  of,  around  granite,  57, 
153. 

Surface  of,  45,  50. 
Quartzite,  Westboro,  155. 
Quartzite,  Worcester,  30-51. 
Quartzose  mica  schist,  130. 
Quinebaug  river,  151. 
Quinsigamond  area,  48. 

Remnants,  44,  144. 

Rhode  Island,  29. 

Ribbing  in  phvllite,  26. 

River  valleys,'  27,  158. 

Rock-floor,  1. 

Rock,  Folding,  crushing,  flowing  of, 

14,  61,  84,  91. 

Rock  material,  Sorting  of,  87. 
Rock  pressure,  60. 

Sand,  deposited  by  water,  32. 

Remnants  of  grains  of,  33. 
Sands,  Thickness  of,  1. 
Scapolite,  109,  112,  154. 
Schist,  see  Brimfield. 
Schist,  Biotite,  84,  88. 

Chloritic,  89. 

Garnetiferous,  95. 

Hornblende,    87,    105,    106,    107, 
113,  114. 

streamers,  85 

structure,  114. 
Secondary  minerals,  96. 


166 


INDEX. 


Secondary  structure,  7,  47,  152. 

Segregation  area,  69. 

Selective  crystallization,  85. 

Sericite,  6. 

Shale,  61. 

Shattering  of  rocks,  14,  17,  37,  42, 

44,  58,  60,  61. 
Shearing  of  rocks,  108,  114. 
Shrewsbury,  154,  155. 
Shrewsbury  dike,  105-115. 
Silliman,  Benjamin,  101. 
Sillimanite  (see  fibrolite),  101. 
Silurian,  29. 

Slaty  cleavage,  8,  26,  46. 
Slic.kensides,  35,  36. 
Solution,  69,  70,  71. 
Sorting  of  rock  material,  87. 
Southboro,  156. 
Sphalerite,  68,  154. 
State  Lunatic  Asylum,  41,  52. 
Staurolite,  22,  27". 
Sterling,  5. 
Stilbite,  97,  134. 

Stone-wall,  Geological  use  of,   13,  14. 
Streamers  of  schist,  85. 
Strength  of  rock,  61. 
Striated  surfaces,  76. 
Strike,  5,  15,  23,  30,  31,  35,  37,  38, 

39,  40,  42,  43,  45,  79,  80,  134, 

137,  138,  141,  145. 
Structure,  7,  26,  42,  47. 
Sturbridge,  150. 
Sulphide  of  zinc,  66,  154. 
Summit,  4,  23,  24. 

Sun's  rays,  Effect  of  on  granite,  75. 
Sutton,  155,  156. 
Swan  farm,  14. 
Swift  river,   151. 
Synclinal  fold,  40,  48,  124. 

Talc,  111. 

Tatnuck,  130. 

Tatnuck  brook  valley,  146. 

Hill  quarry,  131. 
Tertiary,  158. 
Tertiary  valley  of  Worcester,  158. 


Till,  Thickness  of,  1. 

Tourmaline,  130,  133,  135,  137,  142, 

147. 
Tourmaline  granite.    130,    132,    137, 

142,  147. 
Townsend,  151. 
Tremolite,  110,  111. 
Triclinic  feldspar,  53,  63. 

,   Upsala  street,  117. 
j   Upton,  151,  156. 
Uxbridge,  156. 

Van  Hise,  Prof.,  61,  92. 
Vein  of  calcite,  24. 

of  minerals,  73. 

of  quartz,  34,  36,  37,  58,  65,  98, 

100. 

Vermiculite,  99. 
Vernon  street,  4,  117. 

Wachusett,  Mt.,  151,  158. 
Ware  river,  151. 
Warping  in  rock,  47. 
Watatic,  151,  158. 
Weathering,   10,  76,  109.   111. 
Webster,  104,  154,  155. 
Wessonville,  156. 
Westboro,  151,  155,  156. 
Westboro  quartzite,  155. 
West  Bovlston,  5. 

Wigwam' Hill,  14,  15,  44,  47,  48,  79, 
|  127. 

Wilbraham  mountains,  151. 
Wolf  Den  Hill,  5. 
Woodland  street,  4. 
Woodstock,  4. 

Worcester  phyllite,  see  phyllite. 
Worcester,  Physical  Geograjphy  of,  1 . 
Worcester  quartzite,   see   Quartzite, 

micaceous. 

Worcester,  Situation  of,  152. 
Tertiary  valley  of,  159. 

Zinc  sulphide,  66. 
Zircon,  81,  93. 


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